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: If08b49d585c7339878e2239ff7b3e2ea59fd07f8

Author: openstackadmin

tox.ini

@@ -11,7 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
 usedevelop = True
 
-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
 
@@ -51,7 +51,7 @@ commands =
 commands = oslopolicy-sample-generator --config-file=etc/oslo-policy-generator/cloudkitty.conf
 
 [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 --keep-going -b html doc/source doc/build/html