Vagrant.configure is now how configuration is done in Vagrantfiles
(previously it was Vagrant::Config.run). This function takes a single
argument which is the version of configuration to use.
Various internals were updated for this new versioned configuration.
Note that multiple versions of configuration aren't yet used so aren't
fully supported by Vagrant, but the foundation is being set here.
Config::Loader will be the new class responsible for loading configuration
and replaces the previous dual-role "Vagrant::Config" played. While this
commit is very early-stage, once this new architecture is flushed out, it
will make loading, using, and extending configuration much easier and cleaner.
Additionally, I believe this will help post Vagrant 1.0 if multi-language
configuration is implemented.
