r/kubelet: Schedule automatic image prune

As pods move around between nodes, applications are updated, etc., nodes
tend to accumulate images in their container stores that are no longer
used.  These take up space unnecessarily, eventually triggering disk
usage alarms.  From now, the _kubelet_ role installs a systemd timer and
service unit to periodically clean up these unused images.

roles/kubelet/handlers/main.yml

@@ -14,3 +14,8 @@
   service:
     name: kubelet
     state: restarted
+
+- name: restart crictl-image-prune.timer
+  systemd:
+    name: crictl-image-prune.timer
+    state: restarted