Reducing duplicate code on push-image github workflow

[pacostas]

Summary

This PR applies on push-image workflow:

• Refactors the code enabling code re-usability .
• It makes the existing implementation more generic, as a result there will be no changes needed in the future when adding or removing stacks, by dynamically fetching the assets of each release before pushing on dockerhub and GCR.
• Runs jobs in parallel as a result the workflow takes less time to finish.

Use Cases

Checklist

• [x] I have viewed, signed, and submitted the Contributor License Agreement.
• [ ] I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [x] I have added an integration test, if necessary.
• [x] I have reviewed the styleguide for guidance on my code quality.
• [x] I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[pacostas]

The acceptance test failed probably due to the network

[pacostas]

@mhdawson Rebased, resolved the conflicts and commented out the code which pushes to gcr. Is ready for review.

[mhdawson]

@pacostas sorry, accidentally used the merge option to rebase which I did not want. To try to fix I pulled your fork, removed the merge commit and force pushed, which I thought would get me back to the point before I did that. Unfortunately it seems to removed your changes and closed the PR.

I still have a local branch with all of your changes on it, but what probably makes sense is for you to repush your local copy of the branch and re-open the PR, or possibly open an new PR if that is not possble.