Temporary limitation of Vagrant to only allow one active machine with a
provider at a time. That means you cant `up` a machine with both vmware
and virtualbox at the same time. In the future you will be able to but
to avoid various edge cases for now we're disallowing it.
This works by registering a `config` with `:provider => true` with the
same name as your provider. Vagrant will then automatically configure
the provider when `config.vm.provider` is used.
