Update TOX_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: I2a5639f551709c76074f465035752059d3d9ac80

tox.ini

@@ -10,7 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
 passenv = TRACE_FAILONLY GENERATE_HASHES http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 usedevelop = True
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals =