Env variable for explicitly excluding cpython in participating during build

[pacostas]

Summary

This PR:

• introduces a new env variable called BP_NODE_EXCLUDE_BUILD_PYTHON where if is added, will not add the cpython buildpack even if the system does not have the python on the build image, hence no python will be installed on the build image.

Use Cases

Related issue

• Python required for every node app #1329

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).