Update TOX/UPPER_CONSTRAINTS_FILE for stable/ussuri

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/ussuri branch, tests will
continue to use the upper-constraints list on master.

Change-Id: Icaa3eb1c775c787de5c10007a70e44285ac686f9

tox.ini

@@ -6,7 +6,7 @@ ignore_basepython_conflict = true
 
 [testenv]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
   -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
@@ -25,7 +25,7 @@ commands = python setup.py test --coverage --coverage-package-name=oslo_versione
 [testenv:docs]
 whitelist_externals = rm
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build