We supported creating snapshots, but not doing anything with them. This
removes the manual task of restoring a snapshot and replace it with a
PVC abstraction of rolling back to a snapshot.
While Ceph recommends cloning a snapshot instead of rolling back, due to
the time taken, in our usecase I don't think that is an optimal
strategy, as it will leave dangling clones that we'd then have to
manage.
Closes#183