Converted from "login" to "ssh"

I'm a vagrant user, and by default test-kitchen uses vagrant. I kept finding
myself using `bundle exec kitchen ssh` instead of `bundle exec kitchen login`.

I understand that this is only valid for *nix boxes, but being that the main
interface to `kitchen` right now is the command line this seems like a good
option.

For most other drivers you'll be `ssh`ing in also, so this seems like a logical
choice.