[goal] Migrate testing to ubuntu focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing:
- bug#1886298
Bump the lower constraints for required deps which added python3.8 support
in their later version.

- Move multinode jobs to focal nodeset

Story: #2007865
Task: #40227

Closes-Bug: #1886298

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal>

Depends-On: https://review.opendev.org/#/c/752294/

Change-Id: Iec953f3294087cd0b628b701ad3d684cea61c057
(cherry picked from commit e21e5f609e)

.zuul.yaml

@@ -102,7 +102,7 @@
 - job:
     name: watcher-tempest-multinode
     parent: watcher-tempest-functional
-    nodeset: openstack-two-node-bionic
+    nodeset: openstack-two-node-focal
     roles:
       - zuul: openstack/tempest
     group-vars: