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 
Can we also get this added in `lms/envs/aws.py` so that it can be changed via json configuration? Once this is ready to go I can go ahead and add the override option to the configuration repo.