WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
There is a labeling problem here, for sure.

If we use a "cloud provider" to access storage, then we will need to implement a non-cloud implementation for those running in their own datacenter.

NFS shares are one means of storage and it will be slower than a local disk.  NodePersistentDisk is the "fastest" type for that environment.

So, both types are relevant, but the "NFS" label as cloud provider is too limited.   "Enterprise" cloud provider is too vague.