Require assisted_volume_snapshots from novaclient

The "QEMU-assisted-snapshots for GlusterFS volumes" patch currently
does a try/except ImportError for the new assisted_volume_snapshots
extension in novaclient.  That functionality is now in novaclient
2.15.0, so Cinder should consider this a hard requirement rather
than allowing it to be absent.

Change-Id: I5d5b4918f17f26706bed361cbbe8b369b10b0b98
Merge "Require assisted_volume_snapshots from novaclient"