Consumer specific content set/repo overrides

API to maintain overrides on the server.
To be used by client machines and SAM/Katello
Every call returns entire JSON representation of the set of overrides for the specific consumer.
Each add/delete command requires a JSON body.
public schema name unnecessary
alterations for code review
Merge pull request #403 from candlepin/wottop/consumer-content-override

Consumer specific content set/repo overrides