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