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: I7664dda2c66ffb34ceb56ada722574ae0e490f8f
Closes-Bug: 1356083