Refactor Storage Related Resource Code
These changes are a joint effort between Edison and I to refactor some
of the code around snapshotting VM volumes and creating
templates/volumes from VM volume snapshots. In general, we were working
towards allowing PrimaryDataStoreDrivers to create snapshots on primary
storage and not requiring the snapshots to be transferred to secondary
storage.

High level changes:
-Added uuid to NfsTO, SwiftTO & S3TO to cut down on the requirement of
PrimaryDataStoreTO and ImageStoreTO which don't really serve much of a
purpose
-Initial work towards enable reverting VM volume from snapshots
-Added hypervisor commands for introducing and forgetting new hypervisor
objects (snapshots, templates & volumes)

Signed-off-by: Edison Su <sudison@gmail.com>