Update internal values of the MQTT class when configuration is updated

[BabyCakes13]

For now, the link between flask and the configuration is done for mqtt. When the values are updated in the web app, they are also updated in the configuration file.

However, they are not also updated in the MQTT class. Therefore, the old values will always be the ones read for the first time from the configuration file.

I have to implement a method to update the internal values of the MQTT class when the same values are updated in the configuration.

[BabyCakes13]

Done. I have implemented a callback function which is called when the configuration is updated. I kept the implementation abstract, the configuration class has no knowledge of which section is updated or how. It just calls the assigned update function:

in mqtt.py:

in configuration.py

[BabyCakes13]

Done in a81fb8f