Fix setting the tempest virtual env constraints env var

Grenade set the env var TOX_CONSTRAINTS_FILE/UPPER_CONSTRAINTS_FILE
which are used to use the constraints during Tempest virtual env
installation. Those env var are set to non-master constraint when
we need to use non-master constraints but when we need to use the
master constraints we do not set/reset them point to master constraints.

This create the issue when grenade script install the tempest second time
on new devstack where we need to use master constraints but env var
TOX_CONSTRAINTS_FILE/UPPER_CONSTRAINTS_FILE set previously in tempest
installation in old devstack point to the stable constraints.

One case where it started failing when we tried to pin the stable/wallaby with
Tempest 29.0.0
- https://review.opendev.org/c/openstack/devstack/+/871782

and stable/xena grenade job failed (stable/xena use master tempest and supposed
to use master constraints)
- https://zuul.opendev.org/t/openstack/build/fb7b2a8b562c42bab4c741819f5e9732/log/controller/logs/grenade.sh_log.txt#16641

We should set/reset those constraint env var to master constraints if configuration
tell to use the master constraints.

Closes-Bug: #2003993
Change-Id: I8c623008292f3b2dd2cd6d596feea552194acd89

grenade.sh

@@ -308,6 +308,15 @@ if [[ "$RUN_BASE" == "True" ]]; then
             # TOX_CONSTRAINTS_FILE then we can remove the old one.
             export UPPER_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
             export TOX_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
+        else
+            # NOTE(gmann): we need to set the below env var pointing to master
+            # constraints even that is what default in tox.ini. Otherwise it
+            # can create the issue for grenade run where old and new devstack
+            # can have different tempest (old and master) to install. For
+            # detail problem, refer to the
+            # https://bugs.launchpad.net/devstack/+bug/2003993
+            export UPPER_CONSTRAINTS_FILE=https://releases.openstack.org/constraints/upper/master
+            export TOX_CONSTRAINTS_FILE=https://releases.openstack.org/constraints/upper/master
         fi
         tox -esmoke -- --concurrency=$TEMPEST_CONCURRENCY
         if [ "${GRENADE_USE_EXTERNAL_DEVSTACK}" != "True" ]; then