Update TOX_CONSTRAINTS_FILE for stable/wallaby

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

Change-Id: I6a7b8133d72fd96eb71db55ce166a5983d8f459a

tox.ini

@@ -6,7 +6,7 @@ ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/wallaby} {opts} {packages}
 
 [testenv:releasenotes]
 deps = -r{toxinidir}/doc/requirements.txt