ssh: Use the 'os' grain to add Gentoo support for SSH

Since #3019 [1], Portage-based distributions using the ebuild module
[2] (e.g. Gentoo) have required full category/package names.  This commit
adjusts package names appropriately for the 'ssh' state, but similar changes
are likely needed for other states.  The Salt docs suggest using pillars to
configure OS-specific package names [3], but I'm not sure how to integrate
pillar data in this state-oriented repository.

[1]: https://github.com/saltstack/salt/pull/3019
[2]: https://github.com/saltstack/salt/blob/develop/salt/modules/ebuild.py
[3]: http://docs.saltstack.com/topics/pillar/