Automatic merge from submit-queue

separate RESTStorage by API group

This demonstrates how we could start separating the description of `RESTStorage` from `pkg/master`.  Each API group owns constructing its own RESTStorage.  Since `pkg/registry` is where we place all packages that create RESTStorage, it seems reason to split the package by `pkg/registry/<api group>/storage` to create RESTStorage and `pkg/registry/<api group>/<resource>` to support each individual resource.

The interface to construct this RESTStorage is dependent upon some `genericapiserver` types now, but we could (should?) move those items to a different location.

@kubernetes/sig-api-machinery 
@lavalamp can you weigh in on how I've split these packages?  I think we agree that we need better segregation, but we haven't spoken about how to spell it.