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: Ia3d71eaccdfb736068478b935e5be46719eb49db
Closes-Bug: 1356083
(cherry picked from commit 141e65a7ddc07b682fc730bab98bffbd7d49f53f)
Merge "Makes kombu_ssl_* parameters optional when rabbit_use_ssl => true" into stable/icehouse