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: I49dccfab69415af6a6cccc44859ba60b7a71e0b5

tox.ini

@@ -14,7 +14,7 @@ setenv = OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
          OS_TEST_TIMEOUT=60
 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}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
@@ -92,7 +92,7 @@ commands =
 
 [testenv:venv]
 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}/doc/requirements.txt
 commands = {posargs}
@@ -114,7 +114,7 @@ commands =
 
 [testenv: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 =
   sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
@@ -124,7 +124,7 @@ commands =
 
 [testenv:releasenotes]
 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 =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html