Run daily backups of nodepool zk image data

This does local backups of the nodepool zk image image data to
/var/log/nodepool on the nodepool-builders. These hosts don't get
offsite backups but we run mutliple redundant servers. This data isn't
critical as we can start from scratch, but may be useful if we don't
want to go through all that trouble.

Change-Id: I7d150df9c0d9566ef2d32167cea535e29822cfa2

playbooks/roles/nodepool-builder/tasks/main.yaml

@@ -41,3 +41,12 @@
 - name: Run docker prune to cleanup unneeded images
   shell:
     cmd: docker image prune -f
+
+- name: Backup nodepool zk image data daily
+  cron:
+    name: Backup nodepool zk image data
+    minute: '{{ 59 | random(seed=inventory_hostname) }}'
+    hour: '{{ 23 | random(seed=inventory_hostname) }}'
+    # Note we backup to /var/log/nodepool because that is what we have bind
+    # mounted in the container and we are operating relative to that context
+    job: docker exec nodepool-builder-compose_nodepool-builder_1 nodepool export-image-data /var/log/nodepool/nodepool-image-backup.json