Support box_version option

[mconigliaro]

Vagrant has supported box versioning since v1.5. Although it doesn't seem to be well documented, it's actually possible to set box_url to a metadata file rather than a specific box. This PR makes kitchen-vagant support Vagrant's box_version option, without which, Vagrant will default to using the latest version.

This will be very useful to teams who have a large number of proprietary Chef cookbook repositories. Now instead of having to update the .kitchen.yml in every repo for every new base box update, it'll be possible to do global updates through a single metadata file hosted on a server.

[sethvargo]

👍

[sethvargo]

W00t w00t

[mconigliaro]

Thanks for the merge. In the time since I created this, I realized that Vagrant won't actually update the box until you run vagrant box update. You can do this manually of course (e.g. cd .kitchen/kitchen-vagrant/<vm> && vagrant box update), but this is a good example where test-kitchen/test-kitchen#350 would help.