WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
There are already many types of disks implicit in the design because, as you suggested, the developer requests storage via attributes (size, speed, etc).  The provider maps those requested attributes to an appropriate disk type.

The provider can have many types of disks (GCE has 2, AWS has 3, our Foo provider for an enterprise customer has X types).