Switch to the new canonical constraints URL on master
Switch to the new canonical constraints URL on master

Reference:
1. http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html
2. https://github.com/openstack/nova/blob/master/tox.ini#L17

Change-Id: I46c221ad3ffa064a3020017b5f5c8a1da5b228f2

openstack/senlin,openstack/senlin,stackforge/senlin,stackforge/senlin,openstack/senlin