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.

tox.ini is hard coded to use master constraint[1] which force
run-tempest to recreate the tox env and use the master constraint.
Fix for that- https://review.opendev.org/#/c/705870/

nova-live-migration test hook run_test.sh needs to use the stable
u-c so that Tempest installation in venv will use stable
branch constraint.

Modify the irrelevant-files for nova-live-migration job to run
for run_test.sh script.

[1] bc9fe8eca8/tox.ini (L14)

Change-Id: I8190f93e0a754fa59ed848a3a230d1ef63a06abc

.zuul.yaml

@@ -90,6 +90,21 @@
     parent: nova-dsvm-multinode-base
     run: playbooks/legacy/nova-live-migration/run.yaml
     post-run: playbooks/legacy/nova-live-migration/post.yaml
+    irrelevant-files:
+      - ^(placement-)?api-.*$
+      - ^(test-|)requirements.txt$
+      - ^.*\.rst$
+      - ^.git.*$
+      - ^doc/.*$
+      - ^nova/hacking/.*$
+      - ^nova/locale/.*$
+      - ^nova/tests/unit/.*$
+      - ^nova/tests/functional/.*$
+      - ^releasenotes/.*$
+      - ^setup.cfg$
+      - ^tests-py3.txt$
+      - ^tools/.*$
+      - ^tox.ini$
 
 - job:
     name: nova-lvm

nova/tests/live_migration/hooks/utils.sh

@@ -3,7 +3,13 @@
 function run_tempest {
     local message=$1
     local tempest_regex=$2
-    sudo -H -u tempest tox -eall -- $tempest_regex --concurrency=$TEMPEST_CONCURRENCY
+    # NOTE(gmann): Use branch constraint because Tempest is pinned to the branch release
+    # instead of using master. 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=$BASE/new/requirements/upper-constraints.txt
+
+    sudo -H -u tempest UPPER_CONSTRAINTS_FILE=$UPPER_CONSTRAINTS_FILE tox -eall -- $tempest_regex --concurrency=$TEMPEST_CONCURRENCY
     exitcode=$?
     if [[ $exitcode -ne 0 ]]; then
       die $LINENO "$message failure"