adding gui support + vagrant ssh options



Hi !
I know it's a very unusual case, but for my project I need to work with server's GUI.
Currently kitchen-vagrant doesn't support vagrant vm.gui option, so i added this one for virtualbox provider only (default value is false).

Moreover, when you're using a GUI you need to add a hack in vagrant ssh command, so I added vagrant ssh options compatibility (ssh is generic to all providers).
I also tested each options locally with my project, it's ok for me.

Thanks to review and add this :-) !