Allow rabbitmq package to be managed on separately

It seems in the current incarnation if rabbitmq::manage_repos is set to false, the module will still install the EPEL repository, and it wont be compatible with centos/RHEL to allow you to download the rpm from an alternate source. The following change seems to fix that - so we can specify package_source (as our own yum repository) and not have the module install the EPEL repo