launch.py: don't keep creating a new log file in crontab mode

launch.py

@@ -931,8 +931,12 @@ if __name__ == '__main__':
             path_log = os.path.join(os.getcwd(), 'launch_scheduler_log.txt')
         elif not os.path.isabs(options.logfile):
             path_log = os.path.join(os.getcwd(), options.logfile)
-        # log to file and do not print messages
-        output = Logger(open(path_log, 'w'), True)
+        # log to file and do not print messages, but in crontab mode, append!
+        if os.path.exists(path_log) and options.crontab:
+            mode = 'a'
+        else:
+            mode = 'w'
+        output = Logger(open(path_log, mode), True)
         sys.stdout = output
     else:
         path_log = None