QEMU-assisted-snapshots for GlusterFS volumes

Coordinate with Nova to create and delete snaphots for
GlusterFS volumes that are attached to VMs.

Cinder is responsible for creating a QCOW2 file which Nova
will activate in the VM's snapshot chain when a snapshot is
created.

When a snapshot is deleted, Cinder will request for Nova to
perform a block commit/rebase operation to logically delete
the snapshot from the QCOW2 chain.

Implements blueprint qemu-assisted-snapshots

Change-Id: I4a7f0c1bc08d88b0f75d119168dd2077487a62a0
Merge "QEMU-assisted-snapshots for GlusterFS volumes"