Update rabbitmq tasks for ansible 1.7+

In recent versions of Ansible, they no longer support some jinja
patterns in tasks. Specifically, jinja may not *add* arguments to a
module.  Therefore, break each of these tasks into 2 separate cases
handled by the "when" conditional.

!version-patch
Merge pull request #531 from craigtracey/rabbit-ansible

Update rabbitmq tasks for ansible 1.7+