Add tempest integrated storage job to run on ubuntu focal

Based on TC 2023.1 PTG discussion [1] and ML thread[2], we should run
at least one tempest job on the previosly supported Ubuntu version to
make sure that current release can work on older operating system and
that upgrade process can be as smooth as possible for operators.

[1] https://etherpad.opendev.org/p/tc-2023-1-ptg#L428
[2] https://lists.openstack.org/pipermail/openstack-discuss/2022-November/031229.html

Change-Id: I657ff088bb916a8b8085f3cc5b0a53dc493b5af7

.zuul.yaml

@@ -80,6 +80,8 @@
             irrelevant-files: *gate-irrelevant-files
         - openstacksdk-functional-devstack:
             irrelevant-files: *gate-irrelevant-files
+        - tempest-integrated-storage-ubuntu-focal:
+            irrelevant-files: *gate-irrelevant-files
     gate:
       jobs:
         - cinder-grenade-mn-sub-volbak:
@@ -94,6 +96,8 @@
             irrelevant-files: *gate-irrelevant-files
         - openstacksdk-functional-devstack:
             irrelevant-files: *gate-irrelevant-files
+        - tempest-integrated-storage-ubuntu-focal:
+            irrelevant-files: *gate-irrelevant-files
     experimental:
       jobs:
         - cinder-multibackend-matrix-migration:
@@ -191,6 +195,12 @@
             DEFAULT:
               cluster: ceph
 
+- job:
+    name: tempest-integrated-storage-ubuntu-focal
+    description: Job testing for tempest-integrated-storage with Ubuntu focal (20.04)
+    parent: tempest-integrated-storage
+    nodeset: openstack-single-node-focal
+
 - job:
     name: cinder-grenade-mn-sub-bak
     parent: grenade-multinode