Update TOX_CONSTRAINTS_FILE
Update TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is an old and deprecated name, refer [1].

This allows to use lower-constraints file as more readable way instead
of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

[1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file

Change-Id: Ib4dd97f427a0c364e2679c229bda9df6c3155d48

openstack/heat-translator,openstack/heat-translator