The topic names need to be consistent between event types to ensure
people can properly use wildcards to filter the messages on just the
things they care about. However playbook and task were inconsistent in
topics from between different methods in the callback plugin. This
commit fixes that by making sure we always use 'playbook' and 'task',
not 'playbooks' or 'tasks'.
Change-Id: I3e6240560ad562e8f41f7e314ef7a4b0b1178e32
1ef6088bb6