Stack - Select on which registries to push stack images

[pacostas]

Summary

This PR adds an extra option, which specifies on which registries the stack should push. Specifically, if on the root directory of the implementation, there is a file called registries.json with the below content

{
  "dockerhub": true,
  "GCR": false
}

it means that the action will push the stack images to dockerhub but NOT to GCR.

In case the registries.json file does not exist, it fallbacks to the current behavior which is to push on both registries.

In case where an attribute is NOT specified, then the default value is true.

Use Cases

This is useful for projects being under paketo-community organization, where GCR is not available.

Checklist

• 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)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[pacostas]

@mhdawson @ForestEckhardt

[robdimsdale]

This looks good, with one minor question (inline) about potentially duplicated lines.

[pacostas]

@ForestEckhardt @robdimsdale I think this PR is ready to be landed.