Log when a periodic is completed

When debugigng the logs, it is sort of difficult to know when
actions complete for a specifc periodic task.

Example: Power sync, which includes an explicit sleep(0) to
yield control. The explicit sleep *is* important as it is
a low priority task. Where this quickly mixes things up
is when we're hunting database locking issues, and we
*must* verify that we *have exited* part of the code
completely.

Change-Id: Ie9eddedf6bca603845ff14e1ccbda665e3b9e5bd

ironic/conductor/periodics.py

@@ -171,6 +171,7 @@ def node_periodic(purpose, spacing, enabled=True, filters=None,
                 METRICS.send_gauge(
                     node_count_metric_name,
                     node_count)
+            LOG.debug('Completed periodic task for purpose %s.', purpose)
 
         return wrapper