chef_server_url not configurable for upload command

Particularly important for https://github.com/RiotGames/vagrant-berkshelf/issues/39.

When vagrant-berkshelf passes options to Berkshelf, it passes your Vagrantfile's configured chef_server_url. That value never makes it into the Berkshelf configuration used to talk to the Chef server - your ~/.berkshelf/config.json value is preferred.