Improve fail and retry logic when pushing images

[bluesliverx]

Description

This removes the multiprocessing from pushing images, which really isn't needed as we can fail other ways there.

Related Issue

Motivation and Context

I noticed failures when pushing multiple repos from a multiplatform build. Most concerning was that the build acted like everything was ok when it really failed to push at least one of the images.

How Has This Been Tested?

I've added a functional test file, though this doesn't completely exercise the code in question, it should be closer to what actually happens.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.