Replace URL fragments with a single URL

In auth_token we currently specify the URL of the keystone server using a combination of auth_host auth_port auth_protocol etc. This kind of made sense (though unnecessary) when we were using httplib which needed these components individually but this doesn't exist any more and fragments are just annoying to configure and use.

We replace all those fragments with a single config option with an identity_uri which is the full address to the keystone server.