Makes kombu_ssl_* parameters optional when rabbit_use_ssl => true

The kombu_ssl_* parameters should not be required when rabbit_use_ssl => true
Rather, rabbit_use_ssl must be set to true if the kombu_ssl_* parameters are
used.

(Similar to this patch to puppet-neutron: https://review.openstack.org/#/c/113671/ )

Closes-Bug: 1356083
Change-Id: I6a27ebf359c1910e221e9dc31711c6af569dc0ce