Vagrant plugin: Rendering the 'vagrant box (remove|repackage)' completion code independant of Vagrant implementation details.

I was wondering why Vagrant's box-command related command-line completion wasn't working for me; somewhere along the line, Vagrant changed its .d directory from .vagrant to .vagrant.d. I've changed the code to query Vagrant via its publicized command-line interface, thus freeing us from relying on implementation/version-specific details.