move cleanup cronjob to "daily"

tasks/backup.yml

@@ -18,5 +18,4 @@
   cron:
     name: mongodb backup cleanup
     job: 'find /opt/mongodbbackup/ -mindepth 1 -maxdepth 1 -mtime +{{ mongodb.backup.keep_days - 1 }} -type d -exec rm -r {} \;'
-    minute: "0"
-    hour: "5"
+    special_time: daily