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