bundles/backup-client: redirect stderr to /dev/null

We have monitoring to determine wether the backup succeeded or not.
No need for E-Mails, too.

bundles/backup-client/metadata.py

@@ -16,7 +16,7 @@ def cron(metadata):
 
     return {
         'cron': {
-            'backup': '{} 1 * * *    root    /usr/local/bin/generate-backup > /dev/null'.format(
+            'backup': '{} 1 * * *    root    /usr/local/bin/generate-backup > /dev/null 2>&1'.format(
                 (node.magic_number % 60),
             ),
         },