Temporary pin the swift-dsvm-functional nodeset to Focal

As per the community wide goal to migrate the CI/CD from
Ubuntu Focal to Ubuntu jammy, we need to merge the devstack, tox base
jobs to jammy on Nov 18. But swift-dsvm-functional job is
failing on Ubuntu Jammy.

To move ahead to merge the base job patches we need to pin the
swift-dsvm-functional job nodeset to Focal until this is fixed
for Jammy.

Needed-By: https://review.opendev.org/c/openstack/devstack/+/860795

Related-bug: #1996627
Change-Id: Ia35e55782592175a6a9aaafd59d72a0ff8362d61

.zuul.yaml

@@ -280,6 +280,9 @@
     parent: devstack-minimal
     description: |
       Setup a Swift/Keystone environment and run Swift's func tests.
+    # TODO: Remove the nodeset pinning to focal once below bug is fixed
+    # https://bugs.launchpad.net/swift/+bug/1996627
+    nodeset: openstack-single-node-focal
     required-projects:
       - opendev.org/openstack/requirements
       - opendev.org/openstack/swift