Configuring Celery with SSL within Pulp

This PR adds functionality for SSL configuration of Celery along with associated documentation.

Options in server.conf [tasks] are introduced 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.  You can run the tests by running ```nosetests -v test.unit.server.async.test_celery_instance``` from the ```pulp/server/test``` directory.

pulp-qpid-ssl-cfg script has been updated to show similar configuration information for securing Celery.