Unittest case for constraints.py

Change-Id: Ibb37855c675d5def2ce2e1ed387429ce87a4926b
Signed-off-by: Mohammed Junaid <junaid@redhat.com>
Reviewed-on: http://review.gluster.org/5103
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>