Update UPPER_CONSTRAINTS_FILE for stable/stein

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

Change-Id: I08bf9c65c59f808a0b2d813f08ae49886aa81c4a

tox.ini

@@ -7,7 +7,7 @@ skipsdist = True
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
 deps =
-   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/stein}
    -r{toxinidir}/requirements.txt
    -r{toxinidir}/test-requirements.txt
 passenv = OS_TEST_*