site-container: retry image pull

Sometimes pulling an image fails for network hickup, so let's retry 5
times at 5sec interval.

Signed-off-by: Sébastien Han <seb@redhat.com>

site-container.yml.sample

@@ -83,6 +83,10 @@
     # post-tasks for upcoming import -
     - name: "pull {{ ceph_docker_image }} image"
       command: "{{ container_binary }} pull {{ ceph_docker_registry }}/{{ ceph_docker_image }}:{{ ceph_docker_image_tag }}"
+      register: container_pull
+      until: container_pull.rc == 0
+      retries: 5
+      delay: 5
       changed_when: false
       when:
         - is_atomic