WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
What do you suggest for the first iteration of storage?

Do we expose the underlying provider (or otherwise expect a pod author to know it -- a leaky abstraction) and require the actual constant values used in API calls?  Or do we attempt normalize them across all providers to plug the leak?

Low/med/high or fast/fastest are relative and can change, as you suggest, if a new type of disk hits the market.  Suddenly, fastest isn't fastest, but an EBS 'io1' volume is always 'io1'.