Automatic merge from submit-queue

make addition group RESTStorage registration easier

Starts factoring out `RESTStorage` creation to eventually allow for decoupled API group `RESTStorage` configuration.

Right now you can't add additional groups without modifying the main API Group registration in master.go.  Allows the `master.Config` to hold a function that can build a `RESTStorage` based on the `Master` struct.

@lavalamp @caesarxuchao @kubernetes/sig-api-machinery 
@liggitt @smarterclayton