Stop sending json-file by default

By doing this we were over-riding any of the daemon's defaults.
Instead we can send an empty string which docker-py sends on
and the daemon interprets as, 'json-file' as a default if it
hasn't got any other daemon level config options.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

compose/service.py

@@ -657,7 +657,7 @@ class Service(object):
         cap_add = options.get('cap_add', None)
         cap_drop = options.get('cap_drop', None)
         log_config = LogConfig(
-            type=options.get('log_driver', 'json-file'),
+            type=options.get('log_driver', ""),
             config=options.get('log_opt', None)
         )
         pid = options.get('pid', None)