Update docs to configure Celery with SSL.

The documentation previously talked about configuring Qpid and Consumers with
SSL, but did not address configuring Celery with SSL.
Updating output to explain how to also secure Celery.
Add docs on setting the broker_url for Qpid and RabbitMQ.
Adding the ability to configure SSL for Celery.

Introduced options in server.conf [tasks] that allow the necessary SSL
parameters to be specified.  Also an option added that enables SSL for the
celery_instance.

Defaults are added for these new options.  SSL for Celery is disabled by
default, but if enabled the default cert and keys are the default paths
produced by the pulp-qpid-ssl-cfg script.

A test that verifies the proper configuration of the celery_instance with SSL
and without SSL are also added.