generate on branching added extra image: field on jobs pointing to master

[ericvn]

This image overrides the image being updated to the release at the top of each file.

[ericvn]

Still exists for the 1.14 branching.

[ericvn]

Example:

- name: BUILD_WITH_CONTAINER
  value: "0"
image: gcr.io/istio-testing/build-tools:release-1.16-6363b97c6bc60b7a3a8d52c5e08322d8394149e2
jobs:
- command:
  - make
  - build
  env:
  - name: BUILD_WITH_CONTAINER
    value: "0"
  - name: BUILD_WITH_CONTAINER
    value: "0"
  image: gcr.io/istio-testing/build-tools:master-6363b97c6bc60b7a3a8d52c5e08322d8394149e2

You can see the overall master was updated to 1.16, but the individual jobs still have master`.

[ericvn]

You also see BUILD_WITH_CONTAINER appearing twice within a job.

[ericvn]

Would still be good to get this done.