WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
This controller is just one type.  Multiple controllers may satisfy requests for volumes.  Should clarify here that a default CloudProviderVolumeController would handle most creations, and the implementation should allow he possibility of multiple controllers (I could easily have ceph and gce disks, for example)