(MODULES-4011) Add support for Gentoo-based operatingsystems (#810)
- Add support for Gentoo-based operatingsystems
- Add params.pp support for osfamily=Gentoo
- Make postgresql::client management of postgresql-client package optional. (Gentoo does not split client/server/devel/contrib packages). Add spec test for this new behaviour
- Ensure ::server::contrib and ::lib::devel fail on gentoo for same reason
- Extend the systemd-override to Gentoo to propagate port and datadir changes