added gentoo support

- added gentoo support
- added possibility to specify package names for gentoo packages, because there are many overlays with different package names, and official package is missing
- modified package.pp to use package name from params.pp for installation
- added usage of openrc as default init for gentoo
- added initscript for gentoo clients
You could generalize this a bit more to package_name and contrib_package_name and set the default values to $logstash::params::package or $logstash::params::contrib

That will simplify the logic you created in the params.pp file and allow other distro's to specify the package name as well.