cleanup_ironic should not be started on "unstack" phase,
otherwise API service not restarted because auth cache
directory removed.
Change-Id: I3da86b9fb8c3ce1185aff05df0fa83cf259708f4
Also adds an is_ironic_enabled() function to prepare for an upcoming
change in is_service_enabled().
Change-Id: I6e6e0e8b70221e231785ab27e9b5d4836933ac4c