S allius/issue217 (#229)

* move config.py into a sub directory cnf

* adapt unit test

* split config class

- use depency injection to get config

* increase test coverage

app/src/mqtt.py

@@ -5,7 +5,7 @@ import traceback
 
 from modbus import Modbus
 from messages import Message
-from config import Config
+from cnf.config import Config
 from singleton import Singleton
 
 logger_mqtt = logging.getLogger('mqtt')