Adds Persistent Disk volume type.

Implements a GCEPersistentDisk volume type.

Persistent Disks can now be specified in pod manifests and will be mounted and exposed to containers.



(thinking out loud) I wonder if we should consider objects like this to be "extensions" and instead of making a separate entry for each one, allow a single APIObject here, which could be filled with any extension that is understood by the rest of the system. Perhaps for some future change, since it makes validation a bit tricky.