WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
@smarterclayton good feedback overall.  thanks.

AWS offers exactly 3 types of EBS volumes for performance, GCE PD has 2, and NFS shares are completely beyond our control.  Local node storage performance is dependent entirely on the disk and filesystem used on the node.  Durability and HA is dependent on the provider of the storage.

I attempted to normalize the different types of performance so that requests for storage don't have to know the underlying types (a leaky abstraction).

Please let me know how we can achieve what you're asking, because I cannot see it today and certainly not for the first version of storage.