Vagrant: Use vm.box_url when vm.box is not a Vagrant Cloud box

By default a Vagrant Cloud box is used: "chef/ubuntu-14.04".
If the vm.box option does not refer to a Vagrant Cloud box, the vm.box_url option should be used to specify the download location.
This seems like a really weird case. Why not just check if the user defined `vm.box_url` and put it in if they did?