We use Tempest master for testing the supported stable
branches, so using master upper constraints works fine for
such master or stable branch testing, but when we need to
use old Tempest in the below cases then master upper constraints
do not work for Tempest venv:
- Testing Extended Maintenance branch
- Testing py2.7 jobs until stable/train with in-tree tempest plugins
If heat integration test script does not use the master constraints
for Tempest (when any stable branch goes to Extended Maintenance state)
then we need to use the same constraints while running the tests too
otherwise, it will end up recreating the Tempest venv and remove the
already installed tempest plugins and their deps.
This commit set the compatible upper constraint
according the stable branch Tempest version.