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
We will need to create a new service type for openrc so we keep the parts clean and contained.