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: I1af8bffc98f109d04807fb6e746dee7263ea1d47

tox.ini

@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 
@@ -70,7 +70,7 @@ exclude = .venv,.tox,dist,doc,*egg,releasenotes
 
 [testenv:releasenotes]
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html