A few months ago we refactored options to keep it independent of the
implementations, so that it could be used in CLI tools to validate
config or to generate config, without pulling in the full dependency
tree of the master.  This change restores that by separating
server_run_options.go back to its own package.

Also, options structs should never contain non-serializable types, which
storagebackend.Config was doing with runtime.Codec. Split the codec out.

Fix a typo on the name of the etcd2.go storage backend.

Finally, move DefaultStorageMediaType to server_run_options.