Implement initial 'wdcli backup_instance' command

This commit performs an initial implementation of the 'backup_instance'
command.

cmd/system_update.go

@@ -160,6 +160,8 @@ func (si systemupdate) Run(context wisski_distillery.Context) error {
 		return errBootstrapTriplestore.WithMessageF(err)
 	}
 
+	// TODO: Register cronjob in /etc/cron.d!
+
 	logging.LogMessage(context.IOStream, "System has been updated")
 	return nil
 }