Custom cache-key

[odie5533]

Describe Request:

Add cache-key-override to override the built-in cache key. This lets me include files I want so we can properly cache based on whatever I want.

PR: #135

Examples:

      - python/install-packages:
          cache-key-override: "pip-{{ checksum 'dev-requirements.txt' }}-{{ checksum 'requirements.txt' }}

Additional thought

It's very weird that include-branch-in-cache-key: true is true by default. This makes the default caching much less effective. It's a very weird default. If I were designing the default, it would just be a checksum of the incoming pip-dependency-file: requirements.txt.