Add SOCKS support to test_ssh

This pull request adds SOCKS support to Test Kitchen, so that you can spin up EC2 instances from behind a firewall.

This is only a 'partial' pull request, it'll need Pull Request #426 to be included, which adds the ability to add a ProxyCommand argument to the SSH command. That pull request won't work with EC2, unless the changes in this pull request are included as well.

This does introduce a dependency on 'socksify'.

Please let me know if this is a viable pull request. We, along with many others in big enterprise land live behind firewalls. Without this, Test Kitchen is not really all that usable to us.