Conditionaly testing stacks

[pacostas]

Summary

This PR introduces 2 changes:

1. Refactors the code to test the images that exist on images.json file and not from what is specified on the integration.json file
2. Adds functionality to test only the images that exist on TEST_ONLY_STACKS env variable, only if TEST_ONLY_STACKS has been set and is not empty, which is being set from the test.sh script (related PR with the implementation PR Allow testing specific stacks #97)

Related issues

• Investigate running tests on one runtime stack image at a time versus on all stacks at a time #5

Use Cases

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).

[mhdawson]

LGTM