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.

Change-Id: If9cceb4f4090c8b39079aa1aae2f53640c9ac0ff
Closes-Bug: 1356083