WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
Names are easily changed.  I merely dropped the "GCE" from "GCEPersistentDisk" and thought it made sense.

There are many types of volumes and this is just another one.  PersistentVolume is fine, too.  A rose by another name.