Update TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is old name and deprecated
-https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file
This allows to use lower-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

Change-Id: I5e198c592742755f4285b68ac288820b9a33031b
diff --git a/tox.ini b/tox.ini
index 4591a45..07ba459 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@
 [testenv]
 basepython = python3
 usedevelop = True
-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}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning