What does this PR do ?

• For the APM inject task, move the not ansible_check_mode condition to the parent when. We should not run any of the commands/file from dd-host-install in check mode which we were doing individually (except injecting the Docker config which we should not have done in check mode, so this "fixes" it incidentally)
• For the installer task, the playbook "depends" on datadog_installer_install_result which itself is only registered when not running in check mode, as coded in each tasks/pkg-, e.g.:

  ansible-datadog/tasks/pkg-debian/install-installer.yml

  Line 8 in a34e962

  register: datadog_installer_install_result

  . Thus, we add not ansible_check_mode on all the subsequent steps that depend on this registry, as we should not run any bootstrap/installer command, nor attempt to send any telemetry in check mode
• In the main Agent linux task, do not try to check for services that might not exist (e.g. user never ran without --check yet)

Motivation

Closes #610

QA instructions

Use a playbook with the installer:

Role

- hosts: myhosts
  roles:
    - { role: ansible-datadog, become: yes }
  vars:
    datadog_api_key: baguette
    datadog_enabled: false
    datadog_apm_instrumentation_enabled: "host"
    datadog_apm_instrumentation_libraries: ["java:1", "python:2", "js:5", "dotnet:3", "ruby:2"]

Collection

- hosts: myhosts
  tasks:
  - name: Import the Datadog Agent role from the DD collection
    import_role:
      name: datadog.dd.agent
  vars:
    datadog_api_key: baguette
    datadog_enabled: false
    datadog_apm_instrumentation_enabled: "host"
    datadog_apm_instrumentation_libraries: ["java:1", "python:2", "js:5", "dotnet:3", "ruby:2"]

Run it in check mode and ensure it does not error out: ansible-playbook -i inventory.ini playbook.yml --check