WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
> The only use case we *must* support is that I can create a template of an app that has replication controllers and a volume, and there should be no manual intervention required by the developer for the pods to eventually start running attached to a new unique volume created on the fly

I think this is the salient point. What happens under the covers (1. request volume, 2. request pod) is not necessarily identical to the user experience.