Update TOX/UPPER_CONSTRAINTS_FILE for feature/nats-driver

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

Change-Id: I7bef96165091b2b1af1dd0735e70737d31144500

tox.ini

@@ -10,7 +10,7 @@ passenv = OS_*
           REQUIREMENTS_PIP_LOCATION
 install_command = pip install {opts} {packages}
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/nats-driver}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}