Add config.vm.box_server_url setting

This commits adds a new config setting `config.vm.box_server_url` to set
the URL of a local VagrantCloud instance in the Vagrantfile. If the
environment variable `VAGRANT_SERVER_URL` is set, it will still be
preferred.
Merge pull request #4282 from Mayflower/feature/config-box_server_url

core: add config.vm.box_server_url setting