[CHEF-4616] (10-stable) Support IPv6 Literals in `chef_server_url`

Backports the [CHEF-4616](https://tickets.opscode.com/browse/CHEF-4616) patch (https://github.com/opscode/chef/pull/1131) to 10-stable.

Unfortunately, all of the integration test code used in the master/11.x patch is not available in 10.x (it comes from the knife-essentials test code) so all integration testing is done manually. I have verified various APIs including cookbook upload/download against chef-zero bound to ::1.