Add cache-key-override to override cache key logic

[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.

Examples:

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

Issue #134

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Your development orb has been published. It will expire in 30 days.
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