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: Id1837929b52569653ae14db53b995fdb20b0fb21

stackforge/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard