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-ceilometer module it fails if the kombu_* options aren't also
set.

Fixes: rhbz#1141357
Merge pull request #127 from imcsk8/ceilometer-kombu_ssl

Makes kombu_ssl_* parameters optional when rabbit_use_ssl => true