DevStack: Set tempest nameservers config option correctly

The new tempest plugin uses a different group name to the legacy
functionaltests, so we should set both as we transition.

Change-Id: Ie6b0f11399cdcd3d2245872bcd24045e756a5c8a
Depends-On: If524b2d961088745b7e20a279626971c53db9d7c

devstack/plugin.sh

@@ -174,6 +174,9 @@ function configure_designate_tempest() {
             nameservers=$DESIGNATE_NAMESERVERS
         fi
 
+        iniset $TEMPEST_CONFIG dns nameservers $nameservers
+
+        # For legacy functionaltests
         iniset $TEMPEST_CONFIG designate nameservers $nameservers
     fi
 }