Skip the default heroku-postgresql addon if DATABASE_URL is set #1068

Currently the Python buildpack's bin/release step sets a default heroku-postgresql addon if the app has a manage.py script.

However for use-cases where the app wishes to use a different Heroku addon, or a non-Heroku DB, this default addon at best is unused and at worst causes confusion.

To resolve this the buildpack could skip the default addon if DATABASE_URL were already set at the time of the first build.

This would cause the default addon to be skipped if either:

1. the app had another addon attached prior to the first build (that provided the DATABASE_URL env var)
2. DATABASE_URL was manually set (eg via review app apps.json env list) then the default DB would be skipped

For more background see the discussion in:
#1056

Note: The buildpack API does not provide the env dir location during release. As such the env var would need to be checked during bin/compile and either stored until bin/release, or else the release step refactored such that the payload is generated during bin/compile and saved to disk, and then bin/release simply returns the already-generated payload. The latter is what the Ruby buildpack does.