Features/gentoo support

Adds the tweaks needed to enable this module to be deployed on Gentoo-type distributions. Rabbitmq::Params needs a Gentoo block to avoid the compilation fail, and config files/dirs need to be owned by the package-created rabbitmq user to allow rabbitmq-plugin to make changes.
I have no strong preference either way, was just following the convention
set before me. I'll make the change my side if that's the popular opinion.