Use stable constraint for Tempest pinned stable branches

Stable branches till stable/rocky is using python
version <py3.6. Tempest test those branch in venv
but Tempest tox use the master upper-constraint[1]
which block installation due to dependencies
require >=py3.6. For exmaple, oslo.concurrency 4.0.0
is not compatible for <py3.6.

As we pin Tempest for EM stable brach, we should be
able to use stable constraint for Tempest installation
as well as while running during run-tempest playbook.

Old node for grenade job running on stein gate is rocky which
has Tempest pinned so does need stable u-c to use.

[1] bc9fe8eca8/tox.ini (L14)

Change-Id: Iee71472af027994fef4c78cb3e2a524e17e635be

grenade.sh

@@ -281,6 +281,14 @@ if [[ "$RUN_BASE" == "True" ]]; then
     # Validate the install
     if [[ "$BASE_RUN_SMOKE" == "True" ]]; then
         echo_summary "Running base smoke test"
+
+        # NOTE(gmann): This run is on old node which is stable/rocky in stein
+        # case. Use branch constraint because Tempest is pinned on stable/rocky.
+        # We need to export it via env var UPPER_CONSTRAINTS_FILE
+        # so that initial creation of tempest tox use stable branch constraint
+        # instead of master constraint which is hard coded in tempest/tox.ini
+        export UPPER_CONSTRAINTS_FILE=$BASE_RELEASE_DIR/requirements/upper-constraints.txt
+
         cd $BASE_RELEASE_DIR/tempest
         tox -esmoke -- --concurrency=$TEMPEST_CONCURRENCY
         # once we are done, copy our created artifacts to the target