config.vm.box_server_url setting is ignored

https://github.com/mitchellh/vagrant/pull/4282 introduced new setting to set alternate vagrant cloud server URLs, but it doesn't work actually: the value is ignored, and `env[:box_server_url]` in [box_add.rb](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/action/builtin/box_add.rb#L56) is always empty.

This PR helps to process the value correctly.