bundles/backup-client: do backups at 23:xx, so it won't interfere with upgrade-and-reboot

There were still problems with systems starting their backups late in the hour,
but backup servers did upgrade-and-reboot early it the hour. This leads to
incomplete backups, if the machine is rebooting, too.

bundles/backup-client/metadata.py

@@ -19,7 +19,7 @@ def cron(metadata):
 
     return {
         'cron': {
-            'backup': '{} 0 * * *    root    /usr/local/bin/generate-backup > /dev/null 2>&1'.format(
+            'backup': '{} 23 * * *    root    /usr/local/bin/generate-backup > /dev/null 2>&1'.format(
                 (node.magic_number % 60),
             ),
         },