Begin systemizing files in pkg/registry

Files that have RESTStorage implementations now end in "storage", and
files that have registries now end in "registry". I removed some
underscores in file names, since it seems to be go style not to have
them. I split minion_registry.go into two files.

We should consider splitting this package into two, to make more clear
the separation between the layers.

I added two comments but otherwise did not change  any code.