silence mongodb backup cronjob

tasks/backup.yml

@@ -9,7 +9,7 @@
 - name: add backup cronjob for easier backup/restore
   cron:
     name: mongodb backup
-    job: '/usr/bin/mongodump --gzip --archive=/opt/mongodbbackup/$(date "+\%Y\%m\%d-\%H\%M.dump.gz")'
+    job: '/usr/bin/mongodump --gzip --archive=/opt/mongodbbackup/$(date "+\%Y\%m\%d-\%H\%M.dump.gz") 2>/dev/null'
     user: root
     hour: "*/4"
     minute: 45