Update TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is old name and deprecated, 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: I2318835ca129ba973d1086a3d56e506a7400dc7c

Author: manchandavishal

tox.ini

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