Update TOX_CONSTRAINTS_FILE for stable/yoga

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/yoga branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I794db08414564f5c903c1c88e32ac34bcfa16913

Author: openstackadmin

tox.ini

@@ -6,7 +6,7 @@ skipsdist = True
 [testenv]
 install_command = pip install {opts} {packages}
 deps =
-    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
     -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]