Add support for rolling back snapshots

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

api-daemon/pvcapid/helper.py

@@ -2183,6 +2183,22 @@ def ceph_volume_snapshot_rename(zkhandler, pool, volume, name, new_name):
     return output, retcode
 
 
+@ZKConnection(config)
+def ceph_volume_snapshot_rollback(zkhandler, pool, volume, name):
+    """
+    Roll back a Ceph RBD volume to a given snapshot in the PVC Ceph storage cluster.
+    """
+    retflag, retdata = pvc_ceph.rollback_snapshot(zkhandler, pool, volume, name)
+
+    if retflag:
+        retcode = 200
+    else:
+        retcode = 400
+
+    output = {"message": retdata.replace('"', "'")}
+    return output, retcode
+
+
 @ZKConnection(config)
 def ceph_volume_snapshot_remove(zkhandler, pool, volume, name):
     """