Makes kombu_ssl_* parameters optional when rabbit_use_ssl => true

Currently if one tries to enable SSL for AMQP via rabbit_use_ssl in the
puppet-neutron module it fails if the kombu_* options aren't also set.

Fixes: rhbz#1146136
Merge pull request #128 from imcsk8/neutron-kombu_ssl

Makes kombu_ssl_* parameters optional when rabbit_use_ssl => true