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: I9a50d4b091625017134ded3a39c3c44774966c99

tox.ini

@@ -10,7 +10,7 @@ usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 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}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = /bin/rm -f .testrepository/times.dbm
@@ -27,7 +27,7 @@ commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
 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}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -b html doc/source doc/build/html
@@ -45,7 +45,7 @@ exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build
 
 [testenv:releasenotes]
 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}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html