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: I5c93f7e942b1b73023e4e26cdf4a3e5ea957c925

tox.ini

@@ -5,7 +5,7 @@ skipdist = True
 
 [testenv]
 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}
 deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash