Fix retention days

tasks/backup.yml

@@ -17,5 +17,5 @@
 - name: add backup cleanup cronjob
   cron:
     name: mongodb backup cleanup
-    job: 'find /opt/mongodbbackup/* -mtime +{{ mongodb.backup.keep_days }} -type f -exec rm {} \;'
+    job: 'find /opt/mongodbbackup/* -mtime +{{ mongodb.backup.keep_days - 1 }} -type f -exec rm {} \;'
     special_time: daily