Move to VagrantPlugins top-level namespace

Heyo. Was just trying to write a helper function to see when a plugin was installed, and was having issues with vagrant-cachier. Finally realized it was because you're using `module Vagrant` rather than the recommended top-level namespace -- `VagrantPlugins`:

See https://github.com/mitchellh/vagrant-rackspace/blob/master/lib/vagrant-rackspace/action.rb#L5

I don't suppose you'd be willing to change this? PR pending