Fix setting the tempest virtual env constraints env var

Devstack 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 running the grenade
job where we run Tempest on the old devstack as well as in the new devstack.
When tempest is installed on old devstack then old tempest is used and it sets
these env var to stable/<branch> constraints (this is the case when old devstack
(the stable branch is in EM phase) uses the old tempest not the master tempest),
all good till now. But the problem comes when in the same grenade script run
upgrade-tempest install the master tempest (when new devstack branches are in
the 'supported' phase and use the master tempest means) and are supposed to use
the master constraints. But the TOX_CONSTRAINTS_FILE/UPPER_CONSTRAINTS_FILE env
var set by old tempest is used by the tempest and due to a mismatch in constraints
it fails.

This happened when we tried to pin the stable/wallaby with Tempest 29.0.0
- https://review.opendev.org/c/openstack/devstack/+/871782

and table/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 devstack to use the master constraints.

[1] 71c3c40c26/lib/tempest (L124)

Closes-Bug: #2003993
Change-Id: I5e938139b47f443a4c358415d0d4dcf6549cd085

lib/tempest

@@ -128,6 +128,13 @@ function set_tempest_venv_constraints {
         (cd $REQUIREMENTS_DIR &&
             git show master:upper-constraints.txt 2>/dev/null ||
             git show origin/master:upper-constraints.txt) > $tmp_c
+        # 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
     else
         echo "Using $TEMPEST_VENV_UPPER_CONSTRAINTS constraints in Tempest virtual env."
         cat $TEMPEST_VENV_UPPER_CONSTRAINTS > $tmp_c