allow to set client id of mqtt connection in config, to support multi…

[dhavlik]

…ple instances of sungather

[bohdan-s]

Thanks!
I have 2 points:

1. client_id = self.mqtt_config['client_id'] should be self.client_id = self.mqtt_config['client_id']
2. Do you think the default should be SunGather_[Serial] or SunGather_[Model]? That way its unique even if you dont set it?

[dhavlik]

• client_id = self.mqtt_config['client_id'] should be self.client_id = self.mqtt_config['client_id']

Not really. client_id is only used in the line below.

• Do you think the default should be SunGather_[Serial] or SunGather_[Model]? That way its unique even if you dont set it?

This is a good idea, I changed it accordingly (using the model - this way it would still require to set manually if you monitor 2 inverters of the same model).
For security reasons, I would not use the serial number for such things (client id will be logged by the mqtt server).

[bohdan-s]

Thanks for this commit :) will make it easier for people with multiple inverters.