Config for private ssh key for Vagrant

Will make it possible to configure your own private ssh key for vagrant instead of 
the default vagrant private key.

Will set the ssh_key in the Vagrantfile if the ssh_key attribute is set in .kitchen.yml
This didn't work before as in the config.merge(state) the setting in the .kitchen.yml
file is overriden by the setting in the Vagrantfile.

Has also added documentation for this configuration