Update TOX_CONSTRAINTS_FILE for stable/zed

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

Change-Id: I2dc238245aa7eac2d8e884b1104f98298d054211