[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).

- Move the muilinode to focal nodeset.

Story: #2007865
Task: #40195

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

Change-Id: I04d589cd234126f25556139b76205a5afef1529e
Co-Authored-By: Radosław Piliszek <radoslaw.piliszek@gmail.com>

.zuul.yaml

@@ -1,10 +1,10 @@
 - nodeset:
-    name: masakari-openstack-multi-nodes
+    name: masakari-ubuntu-focal-duo
     nodes:
       - name: controller
-        label: ubuntu-bionic
+        label: ubuntu-focal
       - name: compute1
-        label: ubuntu-bionic
+        label: ubuntu-focal
     groups:
       # Nodes running the compute service
       - name: compute
@@ -29,7 +29,7 @@
     parent: devstack
     description: |
       Base multinodes job for devstack-based functional tests
-    nodeset: masakari-openstack-multi-nodes
+    nodeset: masakari-ubuntu-focal-duo
     pre-run: playbooks/devstack/pre.yaml
     run: playbooks/devstack/run.yaml
     post-run: playbooks/devstack/post.yaml