Handle not having the box and providing it via a box_url.

This change allows one to provide a box_url instead of preinstalling the box directly.

The way this was implemented breaks with a terrible runtime error if you don't have the box already installed nor a box_url in your vagrant file. However, this permits you to provide the openstack api_key and auth_url directly in your (remote) box file. This wouldn't be possible if the config was validated prior to handling the box url. The proper way to fix the terrible runtime error would be to check for box_url directly in the HandleBoxUrl method in Vagrant core.

Reviewed-by: Philippe Godin <godp1301@gmail.com>