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: I2b533d9942c823582122036fc85637faa2743d72
Mistral Extra is a library which allows contributors to add optional
functionality to the mistral project, it also contains examples for
which to base new capabilities.