1145723 - log worker startup/shutdown messages in Celery logs

On RHEL6, Pulp's init.d scripts invoke celery in a way that creates log files
under `/var/log/pulp/` that are not used during normal operation. This causes
confusion for users since they think the logs are going to be there instead of
to `/var/log/messages`.

The init.d scripts have been modified to print startup/shutdown messages in
these logs, and direct users to `/var/log/messages` for more information.

I did not add "startup" to the log filenames in case a user is already
collecting logs from their existing locations.
Maybe adjust the wording here too.