Introduced install_class parameter

This module works perfectly, but doesn't fit at least a specific use case (installation of 3.1.4 on Debian osfamily). I'd like to avoid the creation and maintenance of a fork. 
This PR introduces is a quick reusability feature that allows to manage installations of rabbitmq using alternative approaches / sources by providing a custom install_class parameter without modifying the module.
I can refine the PR, for example adding specific spec tests, if you think that such an option might be accepted. Just let me know.