CI: Use the debug stdout callback instead of manual debug

This display in a readable (by humans) way the result of most tasks, and
should be way more readable that what we have now, which is frequently a
bunch of unreadable json.

+ some small fixes (using delegated_to instead of when
  <control_plane_host>)

tests/testcases/015_check-nodes-ready.yml

@@ -21,9 +21,6 @@
     changed_when: false
     register: get_nodes
 
-  - debug:  # noqa name[missing]
-      msg: "{{ get_nodes.stdout.split('\n') }}"
-
   - name: Check that all nodes are running and ready
     command: "{{ bin_dir }}/kubectl get nodes --no-headers -o yaml"
     changed_when: false