Use TOX_CONSTRAINTS_FILE
Use TOX_CONSTRAINTS_FILE

UPPER_CONSTRAINTS_FILE is old name and deprecated
This allows to use upper-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

Change-Id: I68d6faca20e5d8f1523dbd9f3e4f077a2680aa18

openstack/heat,openstack/heat