Make use of `vagrant ssh-config`

Instead of [asking the user](https://github.com/mlafeldt/chef-runner#setting-up-ssh) to set up a static SSH config for each Vagrant machine, use `vagrant ssh-config` to request SSH settings on demand and cache them somewhere.

For this to work properly, the cache needs to be invalidated as soon as the machine's state changes (e.g. via `vagrant reload`). Not sure how to accomplish that without some sort of state machine. Ideas are welcome.

Update: One way to solve the caching problem might be to ask Vagrant for the ssh config once and then query VirtualBox for the current SSH port (forwarding). See https://github.com/dergachev/vgrnt/blob/master/lib/vgrnt/util/virtualbox.rb