Configure the Tempest venv upper constraints properly

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

grenade run smoke test on old devstack and if old devstack does not
use the master constraints for Tempest then we need to take care of
the same while running the tests too otherwise, it will end up recreating
the Tempest venv and flush the already installed tempest plugins
and their deps.

This commit set the compatible upper constraint
according to TEMPEST_VENV_UPPER_CONSTRAINTS variable, which is
what used in devstack while creating the Tempest venv and install
tempest plugins in that venv.

Closes-Bug: 1922597
Change-Id: I66b5d128c3dbe51289410760ea1cbf7f61c55c2f

grenade.sh

@@ -39,6 +39,8 @@ export DSTOOLS_VERSION=${DSTOOLS_VERSION:-0.4.0}
 source $GRENADE_DIR/grenaderc
 source $GRENADE_DIR/inc/bootstrap
 
+source $BASE_DEVSTACK_DIR/stackrc
+
 while getopts bqs:t c; do
     case $c in
         b)
@@ -289,6 +291,20 @@ if [[ "$RUN_BASE" == "True" ]]; then
     if [[ "$BASE_RUN_SMOKE" == "True" ]]; then
         echo_summary "Running base smoke test"
         cd $BASE_RELEASE_DIR/tempest
+
+        # NOTE(gmann): If gate explicitly set the non master
+        # constraints to use for Tempest venv then use the same
+        # while running the tests too otherwise, it will recreate
+        # the Tempest venv due to constraints mismatch.
+        # recreation of Tempest venv can flush the initially installed
+        # tempest plugins and their deps.
+        if [[ "$TEMPEST_VENV_UPPER_CONSTRAINTS" != "master" ]]; then
+            echo "Using $TEMPEST_VENV_UPPER_CONSTRAINTS constraints in Tempest virtual env."
+            # NOTE: setting both tox env var and once Tempest start using new var
+            # TOX_CONSTRAINTS_FILE then we can remove the old one.
+            export UPPER_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
+            export TOX_CONSTRAINTS_FILE=$TEMPEST_VENV_UPPER_CONSTRAINTS
+        fi
         tox -esmoke -- --concurrency=$TEMPEST_CONCURRENCY
         if [ "${GRENADE_USE_EXTERNAL_DEVSTACK}" != "True" ]; then
             # once we are done, copy our created artifacts to the target