Replace deprecated UPPER_CONSTRAINTS_FILE variable

UPPER_CONSTRAINTS_FILE is deprecated and TOX_CONSTRAINTS_FILE is
the new environment variable name that replaces it [1].

This allows to use upper-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
[2] https://review.opendev.org/#/c/722814/

Change-Id: I57b06800075922b1cd765cad7d717b85d3f5cf17

tox.ini

@@ -6,7 +6,7 @@ ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 
 [testenv:releasenotes]
 deps = -r{toxinidir}/doc/requirements.txt