WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
Fair enough.  I see the perf traits for the various disks in GCE and AWS.  They don't match each others (in terms of values), but they have consistent types (IOPS, throughput, etc), so I can make an internal mapping from your requested disk to what the provider offers.