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: Ib14dc581bbe3fbc38aa849f052543a05de2d3e01

tox.ini

@@ -9,7 +9,7 @@ basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
-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}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -35,7 +35,7 @@ commands =
 
 [testenv:docs]
 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 =
@@ -51,7 +51,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,.ropeproject,tools,hori
 
 [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 -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html