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 prefer config :osfamily, it's a little more explicit and we don't know if there are other Linuxes that this won't work on.