Add support for ordering in ListCommand

Make list command order results either by 'order_by' class attribute,
or by primary key.

snf-webproject/synnefo/webproject/management/commands/__init__.py

@@ -104,6 +104,8 @@ class ListCommand(BaseCommand):
     # Default filters and excludes
     filters = {}
     excludes = {}
+    # Order results
+    order_by = None
 
     # Fields used only with user_user_field
     astakos_url = None
@@ -240,6 +242,9 @@ class ListCommand(BaseCommand):
         except Exception as e:
             raise CommandError("Can not filter results: %s" % e)
 
+        order_key = self.order_by if self.order_by is not None else 'pk'
+        objects = objects.order_by(order_key)
+
         # --display-mails option
         display_mails = options.get("display_mails")
         if display_mails: