download/container: adapt to single key repo+tag -> image_name

This code will be deleted for the rework of the download role, for now
we minimally adapt it to the new structure of downloads.

roles/download/tasks/prep_kubeadm_images.yml

@@ -42,8 +42,7 @@
       value:
         enabled: true
         container: true
-        repo: "{{ item | regex_replace('^(.*):.*$', '\\1') }}"
-        tag: "{{ item | regex_replace('^.*:(.*)$', '\\1') }}"
+        image_name: "{{ item }}"
         groups:
           - k8s_cluster
   loop: "{{ kubeadm_images_list | flatten(levels=1) }}"