Add ability to configure python version in base image

[gerrod3]

Big refactor in the CI. I've broken up pulp_images.yml into multiple components:

• The CI tests for pull-requests -> ci.yml
• The release machinary -> release.yml

The common actions between each run (building images and publishing) have been converted to actions

• Building the base images -> actions/base_images
• Building an app image -> actions/build_image
• Publishing to the registries -> actions/publish_images

There is still a big overlap in ci.yml & release.yml that I want to remove, but I've been writing this for a while so I need to get some feedback before trying to make it perfect.

[gerrod3]

Got the tests to pass, but I am not sure if the release machinary will work. Need to double-check to make sure the tagging logic is correct.