Create env vars to override the full worker image ref

[carbonin]

This will allow advanced users more flexability in how
the worker images are specified. Specifically this allows
changing the image name itself and also allows for digest-based
identifiers (rather than using only tags)

Applies to ManageIQ/manageiq-pods#557

[miq-bot]

Checked commit carbonin@3b51664 with ruby 2.5.7, rubocop 0.69.0, haml-lint 0.28.0, and yamllint
4 files checked, 0 offenses detected
Everything looks fine. 👍

[simaishi]

Jansa backport details:

$ git log -1
commit e33a8f3fd8665356334b5c61976fed84902d3bc0
Author: Jason Frey <fryguy9@gmail.com>
Date:   Fri Jun 19 15:09:47 2020 -0400

    Merge pull request #20297 from carbonin/env_for_full_image_ref

    Create env vars to override the full worker image ref

    (cherry picked from commit 76aefb0bee8c834a6d26c609d66e87599adb0f78)