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/
Merge pull request #37 from wking/gentoo-ssh

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