Update the constraints url

For more detail, please refer:
http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html

Change-Id: I5af911e4467f2dff329cee5b8954ddbb024a7373

tox.ini

@@ -6,7 +6,7 @@ skipsdist = True
 [testenv]
 usedevelop = True
 whitelist_externals = rm
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 setenv = VIRTUAL_ENV={envdir}