refactor(travis): merge lint stage into the test stage

* Runs the lint concurrently, for quicker feedback and time savings
* Some of the restructuring inspired from:
  - https://github.com/travis-ci/travis-yml/blob/5ad02354de7d4fc3e9b9f71c44902058402d45ef/spec/fixtures/configs/co/coolo:open-build-service.yml

.travis.yml

@@ -1,73 +1,34 @@
 # -*- coding: utf-8 -*-
 # vim: ft=yaml
 ---
+# Machine config
 dist: bionic
-stages:
-  - test
-  - lint
-  - name: release
-    if: branch = master AND type != pull_request
-
 sudo: required
-cache: bundler
-language: ruby
-
 services:
   - docker
 
-# Make sure the instances listed below match up with
-# the `platforms` defined in `kitchen.yml`
-# NOTE: Please try to select up to six instances that add some meaningful
-#       testing of the formula's behaviour. If possible, try to refrain from
-#       the classical "chosing all the instances because I want to test on
-#       another/all distro/s" trap: it will just add time to the testing (see
-#       the discussion on #121).  As an example, the set chosen below covers
-#       the most used distros families, systemd and non-systemd and the latest
-#       three supported Saltstack versions with python2 and 3.
-#       As for `kitchen.yml`, that should still contain all of the platforms,
-#       to allow for comprehensive local testing
-#       Ref: https://github.com/saltstack-formulas/template-formula/issues/118
-#       Ref: https://github.com/saltstack-formulas/template-formula/issues/121
-env:
-  matrix:
-    - INSTANCE: default-debian-10-develop-py3
-    # - INSTANCE: default-ubuntu-1804-develop-py3
-    # - INSTANCE: default-centos-7-develop-py3
-    # - INSTANCE: default-fedora-30-develop-py3
-    # - INSTANCE: default-opensuse-leap-15-develop-py3
-    # - INSTANCE: default-amazonlinux-2-develop-py2
-    # - INSTANCE: default-arch-base-latest-develop-py2
-    # - INSTANCE: default-debian-9-2019-2-py3
-    - INSTANCE: default-ubuntu-1804-2019-2-py3
-    # - INSTANCE: default-centos-7-2019-2-py3
-    # - INSTANCE: default-fedora-30-2019-2-py3
-    # - INSTANCE: default-opensuse-leap-15-2019-2-py3
-    - INSTANCE: default-amazonlinux-2-2019-2-py2
-    - INSTANCE: default-arch-base-latest-2019-2-py2
-    # - INSTANCE: default-debian-9-2018-3-py2
-    # - INSTANCE: default-ubuntu-1604-2018-3-py2
-    # - INSTANCE: default-centos-7-2018-3-py2
-    - INSTANCE: default-fedora-29-2018-3-py2
-    - INSTANCE: default-opensuse-leap-15-2018-3-py2
-    # - INSTANCE: default-amazonlinux-2-2018-3-py2
-    # - INSTANCE: default-arch-base-latest-2018-3-py2
-    # - INSTANCE: default-debian-8-2017-7-py2
-    # - INSTANCE: default-ubuntu-1604-2017-7-py2
-    - INSTANCE: centos6-centos-6-2017-7-py2
-    # - INSTANCE: default-fedora-29-2017-7-py2
-    # - INSTANCE: default-opensuse-leap-15-2017-7-py2
-    # - INSTANCE: default-amazonlinux-2-2017-7-py2
-    # - INSTANCE: default-arch-base-latest-2017-7-py2
+# Language and cache config
+language: ruby
+cache: bundler
 
+# Script to run for the test stage
 script:
   - bin/kitchen verify ${INSTANCE}
 
+# Stages and jobs matrix
+stages:
+  - test
+  - name: release
+    if: branch = master AND type != pull_request
 jobs:
   include:
-    # Define the `lint` stage (runs `yamllint` and `commitlint`)
-    - stage: lint
-      language: node_js
+    # Define the test stage matrix
+    # Run all of the linters in a single instance (concurrently with the main tests)
+    # Define the rest of the matrix based on Kitchen testing
+
+    - language: node_js
       node_js: lts/*
+      env: 'Lint: yamllint & commitlint'
       before_install: skip
       script:
         # Install and run `yamllint`
@@ -78,6 +39,49 @@ jobs:
         - npm install @commitlint/config-conventional -D
         - npm install @commitlint/travis-cli -D
         - commitlint-travis
+
+    # Make sure the instances listed below match up with
+    # the `platforms` defined in `kitchen.yml`
+    # NOTE: Please try to select up to six instances that add some meaningful
+    #       testing of the formula's behaviour. If possible, try to refrain from
+    #       the classical "chosing all the instances because I want to test on
+    #       another/all distro/s" trap: it will just add time to the testing (see
+    #       the discussion on #121).  As an example, the set chosen below covers
+    #       the most used distros families, systemd and non-systemd and the latest
+    #       three supported Saltstack versions with python2 and 3.
+    #       As for `kitchen.yml`, that should still contain all of the platforms,
+    #       to allow for comprehensive local testing
+    #       Ref: https://github.com/saltstack-formulas/template-formula/issues/118
+    #       Ref: https://github.com/saltstack-formulas/template-formula/issues/121
+    - env: INSTANCE=default-debian-10-develop-py3
+    # - env: INSTANCE=default-ubuntu-1804-develop-py3
+    # - env: INSTANCE=default-centos-7-develop-py3
+    # - env: INSTANCE=default-fedora-30-develop-py3
+    # - env: INSTANCE=default-opensuse-leap-15-develop-py3
+    # - env: INSTANCE=default-amazonlinux-2-develop-py2
+    # - env: INSTANCE=default-arch-base-latest-develop-py2
+    # - env: INSTANCE=default-debian-9-2019-2-py3
+    - env: INSTANCE=default-ubuntu-1804-2019-2-py3
+    # - env: INSTANCE=default-centos-7-2019-2-py3
+    # - env: INSTANCE=default-fedora-30-2019-2-py3
+    # - env: INSTANCE=default-opensuse-leap-15-2019-2-py3
+    - env: INSTANCE=default-amazonlinux-2-2019-2-py2
+    - env: INSTANCE=default-arch-base-latest-2019-2-py2
+    # - env: INSTANCE=default-debian-9-2018-3-py2
+    # - env: INSTANCE=default-ubuntu-1604-2018-3-py2
+    # - env: INSTANCE=default-centos-7-2018-3-py2
+    - env: INSTANCE=default-fedora-29-2018-3-py2
+    - env: INSTANCE=default-opensuse-leap-15-2018-3-py2
+    # - env: INSTANCE=default-amazonlinux-2-2018-3-py2
+    # - env: INSTANCE=default-arch-base-latest-2018-3-py2
+    # - env: INSTANCE=default-debian-8-2017-7-py2
+    # - env: INSTANCE=default-ubuntu-1604-2017-7-py2
+    - env: INSTANCE=centos6-centos-6-2017-7-py2
+    # - env: INSTANCE=default-fedora-29-2017-7-py2
+    # - env: INSTANCE=default-opensuse-leap-15-2017-7-py2
+    # - env: INSTANCE=default-amazonlinux-2-2017-7-py2
+    # - env: INSTANCE=default-arch-base-latest-2017-7-py2
+
     # Define the release stage that runs `semantic-release`
     - stage: release
       language: node_js