Improvements

* `vagrant destroy` now works - Machine must stopped state before they're destroyed.
* add confirmation support to `vagrant destroy`
* use keyname/keyfile for all authentication - Based on a few conversations at #ChefConf and the itchiness of my beard we should just act as if password auth does not exist! =)
* fall back to machine name if `node_name` not set by user
* display `joyant_keyname` when instance is created - This makes more sense then the ssh username and follows the pattern of other Vagrant cloud providers like `vagrant-aws`.