Field Overrides: Work in Progess

We just wanted to start a place for discussion of this feature as it evolves.  

Primarily for @cpennington @carsongee @pdpinch @davisagli 
Could you also add this to envs/aws.py? It makes it easy for us to configure it in our playbook vars.

For example:
`FIELD_OVERRIDE_PROVIDERS = tuple(ENV_TOKENS.get('FIELD_OVERRIDE_PROVIDERS', []))`
..somewhere within the "NON-SECURE ENV CONFIG" section.