Always direct backup even when volume is in-use

[c-mart]

Since OpenStack Liberty, Cinder has supported non-disruptive volumes of in-use volumes. We don't need to take a separate snapshot, create a temporary volume from that snapshot, and back that up anymore. This change passes the force=True flag when the volume is created, so that Cinder will always back up the volume even if it is in use.

This will also make Ceph incremental backups work again (because subsequent backups will be backing up the same volume rather than different volumes).

~(Note that right now this PR also contains these commits; will rebase against master before removing WIP in title.)~