Update TOX_CONSTRAINTS_FILE for stable/xena

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/xena branch, tests will
continue to use the upper-constraints list on master.

Change-Id: Ibf954db27eff8799ccd1fe43454041d90a22c780

tox.ini

@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = False
 whitelist_externals = rm
-install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
@@ -36,7 +36,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
        -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html