[BaseController::Logger] Use miq_user_role.name

While `virtual_delegates` can be used to improve performance via a
single attribute in a query, we already bring back the `miq_user_role`
(via the lookup_scope), so it is better to just use that.

And currently virtual_delegates don't support already loaded
relationships.

app/controllers/api/base_controller/logger.rb

@@ -23,7 +23,7 @@ def log_api_auth
           group = User.current_user.current_group
           log_request("Authorization", :user   => User.current_user.userid,
                                        :group  => group.description,
-                                       :role   => group.miq_user_role_name,
+                                       :role   => group.miq_user_role.name,
                                        :tenant => group.tenant.name)
         end
       end