WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
2 is an impossible number.  Currently, there is one provider of infrastructure (the cloud providing the disks).  Do we intend for there to be many types of infrastructure for storage in the same environment?  Can't mix the cloud providers, of course, but do we need to support having NFS shares *and* a GCEPersistentDisk in the same cluster?  I cannot imagine a good use case for this.