WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
> Do we need to support having NFS shares and a GCEPersistentDisk in the same cluster? I cannot imagine a good use case for this.

How about the use case where the cluster administrators want to create a GCE persistent disk or EC2 EBS volume and then use that to serve up NFS shares to users?