Fix the format of the list of images extracted from kubeadm (#11741)

The download role expect 'groups' be a list, in order to properly filter
images.

roles/download/tasks/prep_kubeadm_images.yml

@@ -44,7 +44,8 @@
         container: true
         repo: "{{ item | regex_replace('^(.*):.*$', '\\1') }}"
         tag: "{{ item | regex_replace('^.*:(.*)$', '\\1') }}"
-        groups: k8s_cluster
+        groups:
+          - k8s_cluster
   loop: "{{ kubeadm_images_list | flatten(levels=1) }}"
   register: kubeadm_images_cooked
   run_once: true