Update TOX_CONSTRAINTS_FILE
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

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