Adding BP_UBI_RUN_IMAGE_OVERRIDE environment variable

[pacostas]

Merge After

@paketo-bot-reviewer

• fix: integration tests, node home validation should be empty string #82

Summary

This PR introduces an new environment variable called BP_UBI_RUN_IMAGE_OVERRIDE. The purpose of this env variable, is to set the run image with the value of BP_UBI_RUN_IMAGE_OVERRIDE.

On the generate phase of the extension, is specified which image should be set as a run image. With this PR, we are able to explicitly set the run image according to the value of BP_UBI_RUN_IMAGE_OVERRIDE. As a result if the BP_UBI_RUN_IMAGE_OVERRIDE environment variable has been set and is not an empty string, the run image will have the value of BP_UBI_RUN_IMAGE_OVERRIDE otherwise is determined on how it used to be in the past, based on the node version.

Use Cases

This change, helps us on setting run images without modifying the source code of the extension (the extension.toml file). E.x. when we release a new stack, we dont have the option to test the release candidate stack without modifying the source code of the extension, as the run image is not published yet to a registry. By explicitly specifying the run image with the BP_UBI_RUN_IMAGE_OVERRIDE, we are able to test that run image, before creating a release and pushing it to a registry.

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]

Overall LGTM, just need to add space after // in comment lines. Believe that is what is used in existing comments (at least from my look at the nodejs-engine)

[pacostas]

@paketo-bot-reviewer Tests are failing as the #82 hasnt been merged yet.

[thitch97]

@pacostas this all makes sense to me. Happy to merge it in once the merge conflicts are resolved