letsencrypt test : update to jammy

Update this test to use Jammy nodes.  Also add a bit more of a comment
about what's going on.

Change-Id: Ia0c5b922ba70b4ef6c670081f8957fadbd8b3399

zuul.d/system-config-run.yaml

@@ -225,16 +225,24 @@
     name: system-config-run-letsencrypt
     parent: system-config-run
     description: |
-      Run the playbook for letsencrypt key acquisition
+      Run the playbook for letsencrypt key acquisition.
     nodeset:
       nodes:
         - <<: *bridge_node_x86
+        # The other tests run the letsencrypt.yaml playbook to create
+        # self-signed certificates but do not exercise any of the DNS
+        # path because they don't have DNS servers in the inventory.
+        # By adding them for this test, the letsencrypt.yaml playbook
+        # will make a request to LE staging and get TXT records, which
+        # it will populate to the test DNS servers.  LE won't actually
+        # authenticate those records, but we are validating the path
+        # of at least creating and collecting them here.
         - name: adns-letsencrypt.opendev.org
-          label: ubuntu-bionic
+          label: ubuntu-jammy
         - name: letsencrypt01.opendev.org
-          label: ubuntu-bionic
+          label: ubuntu-jammy
         - name: letsencrypt02.opendev.org
-          label: ubuntu-bionic
+          label: ubuntu-jammy
       groups:
         - <<: *bastion_group
     vars: