If delete default qos group, return error message
If delete default qos group, return error message

If admin want to delete default tenant Qos group,
an error message "Default Project QoS Group can not be deleted."
will be return by api.

Change-Id: Ic7b7e74437f095f87d1e2295c619d9c5b02c8c61

TieWei/nova,TieWei/nova