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

daemon-common/ceph.py

@@ -1082,6 +1082,36 @@ def rename_snapshot(zkhandler, pool, volume, name, new_name):
     )
 
 
+def rollback_snapshot(zkhandler, pool, volume, name):
+    if not verifyVolume(zkhandler, pool, volume):
+        return False, 'ERROR: No volume with name "{}" is present in pool "{}".'.format(
+            volume, pool
+        )
+    if not verifySnapshot(zkhandler, pool, volume, name):
+        return (
+            False,
+            'ERROR: No snapshot with name "{}" is present for volume "{}" in pool "{}".'.format(
+                name, volume, pool
+            ),
+        )
+
+        # 1. Roll back the snapshot
+        retcode, stdout, stderr = common.run_os_command(
+            "rbd snap rollback {}/{}@{}".format(pool, volume, name)
+        )
+        if retcode:
+            return (
+                False,
+                'ERROR: Failed to roll back RBD volume "{}" in pool "{}" to snapshot "{}": {}'.format(
+                    volume, pool, name, stderr
+                ),
+            )
+
+    return True, 'Rolled back RBD volume "{}" in pool "{}" to snapshot "{}".'.format(
+        volume, pool, name
+    )
+
+
 def remove_snapshot(zkhandler, pool, volume, name):
     if not verifyVolume(zkhandler, pool, volume):
         return False, 'ERROR: No volume with name "{}" is present in pool "{}".'.format(