Support Test Kitchen instances

~~chef-runner could auto-detect whether the specified Vagrant machine is a Test Kitchen instance, e.g.~~

```
$ chef-runner -M default-ubuntu-1204
```

Alternatively, we could try to support all types of Test Kitchen instances:

```
$ chef-runner -K default-ubuntu-1204
```

TODO:

* [ ] Figure out whether this really works with all TK instances reachable over SSH.
* [x] Write documentation.