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: Ic1b791ae72a77ee716983b2cc4dd1903b5ce4613

tox.ini

@@ -12,7 +12,7 @@ setenv =
     OS_STDERR_CAPTURE=1
     OS_TEST_TIMEOUT=60
     OS_LOG_CAPTURE=1
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}