Fixes #6995: restore pulp_celerybeat to list of services to restart.

The service pulp_celerybeat was removed from the list of services
restarted with a katello:reset during the pulp 2.4 upgrade. Restarting
the service is working fine now so this commit restores pulp_celerybeat
to the list of services to start/stop during katello:reset.

http://projects.theforeman.org/issues/6995