[stable-only]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: I8a7d2b1724fd479fa7e530a5a670167f741841f4

Author: openstackadmin

tox.ini

@@ -24,7 +24,7 @@ setenv =
   OS_TEST_TIMEOUT=160
   PYTHONDONTWRITEBYTECODE=1
 deps =
-  -c{env:TOX_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
 passenv =
@@ -179,7 +179,7 @@ description =
 # Note that we don't use {[testenv]deps} for deps here because we don't want
 # to install (test-)requirements.txt for docs.
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build/html doc/build/doctrees