Switch devstack base job to Ubuntu Focal

As per Victoria cycle testing runtime[1], we need
to test upstream CI/CD on Ubuntu 20.04(Focal). TC is
in the process of defining this as a community goal[2]

By moving the devstack base job to Ubuntu focal, all project side job
or devstack-tempest job will automatically migrate to the new distro
until they override the nodeset.

As devstack is branched, all the stable jobs derived from devstack jobs will
keep working on Bionic.

[1] https://governance.openstack.org/tc/reference/runtimes/victoria.html
[2] https://governance.openstack.org/tc/goals/proposed/migrate-ci-cd-jobs-to-ubuntu-focal.html

Depends-On: https://review.opendev.org/#/c/738328/

Story: #2007865
Task: #40212

Change-Id: I63e69e898376e728eaf6d857426276fc93a1a0e3

.zuul.yaml

@@ -391,7 +391,7 @@
     description: |
       Minimal devstack base job, intended for use by jobs that need
       less than the normal minimum set of required-projects.
-    nodeset: openstack-single-node-bionic
+    nodeset: openstack-single-node-focal
     required-projects:
       - opendev.org/openstack/requirements
     vars:
@@ -565,7 +565,7 @@
 - job:
     name: devstack-multinode
     parent: devstack
-    nodeset: openstack-two-node-bionic
+    nodeset: openstack-two-node-focal
     description: |
       Simple multinode test to verify multinode functionality on devstack side.
       This is not meant to be used as a parent job.
@@ -589,12 +589,10 @@
     nodeset: devstack-single-node-opensuse-15
 
 - job:
-    name: devstack-platform-focal
+    name: devstack-platform-bionic
     parent: tempest-full-py3
-    description: Ubuntu Focal Fossa platform test
-    nodeset: openstack-single-node-focal
-    vars:
-      tempest_black_regex: "(tempest.api.compute.volumes.test_attach_volume.AttachVolumeMultiAttachTest.test_resize_server_with_multiattached_volume|tempest.api.compute.servers.test_server_rescue_negative.ServerRescueNegativeTestJSON|tempest.api.compute.servers.test_server_rescue.ServerStableDeviceRescueTest.test_stable_device_rescue_disk_virtio_with_volume_attached)"
+    description: Ubuntu Bionic platform test
+    nodeset: openstack-single-node-bionic
     voting: false
 
 - job:
@@ -688,7 +686,7 @@
         - devstack-platform-opensuse-15
         - devstack-platform-fedora-latest
         - devstack-platform-centos-8
-        - devstack-platform-focal
+        - devstack-platform-bionic
         - devstack-multinode
         - devstack-unit-tests
         - openstack-tox-bashate