Initialize logging in initialization.py.

Logging used to be initialized in application.py, but this meant that
the Celery workers weren't using Pulp's logging configuration. This
commit moves the logging initialization into the common
initialization.py module.

This commit also removes the --loglevel arguments from our systemd unit
files for Celery. We now capture all Celery logs using Pulp's logging
configuration, except for early Celery startup logs that will still be
captured over stdout by systemd.
Remove systemd celery log level settings.

Now that we are capturing our logs with our Pulp logging.cfg, we no
longer need to have systemd also capture the stdout output from Celery.
This commit removes these settings from the /etc/default/pulp_* files.