Find and install box file for multi-provider boxes automatically

That's a simple change with a quite long explanation.

I develop multi-provider boxes, and publish them on private Vagrant cloud server.
For that I need to put all vSphere connection settings into "dummy box" file, which is published on the server.
Actual Vagrantfiles inherit these settings, and do not set them explicitly.
The setup works fine, with one exception: on each new machine `vagrant up` fails with errors like `Configuration must specify a vSphere host`. This happens because dummy box files are not downloaded yet, and vSphere connections settings are nowhere defined. I have to explicitly use `vagrant box add` command to download the box file from the server.

This pull request fixes the whole issue. It forces Vagrant to check whether required box file for this particular provider is installed, and if not then downloads and installs it automatically.