Set TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is deprecatedi[1], set the newer
TOX_CONSTRAINTS_FILE as well when interacting with other repos for the
transition.

Update internal use but not the tox.ini file, since users calling it
might use the old variable.

[1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file

Change-Id: Iea09402cf918a6978120a341950643185c94ec85
diff --git a/tools/tempest-plugin-sanity.sh b/tools/tempest-plugin-sanity.sh
index b484a41..2ff4aea 100644
--- a/tools/tempest-plugin-sanity.sh
+++ b/tools/tempest-plugin-sanity.sh
@@ -60,8 +60,8 @@
     fi
 }
 
-: ${UPPER_CONSTRAINTS_FILE:="https://releases.openstack.org/constraints/upper/master"}
-DEPS="-c${UPPER_CONSTRAINTS_FILE}"
+: ${TOX_CONSTRAINTS_FILE:="https://releases.openstack.org/constraints/upper/master"}
+DEPS="-c${TOX_CONSTRAINTS_FILE}"
 
 # function to create virtualenv to perform sanity operation
 function prepare_workspace {