allow a mnemonic MQTT topic instead of hardcoded chn<number>

[Zugschlus]

This backwards-compatible change allows one to set the mqtt topic from the channel configuration. Currently, the meter values are published via an mqtt topic that is set to the channel number, which changes whenever a new channel is defined or and old deleted. And it's totally not mnemonic.

Configuration example:

   "meters": [
        { 
            // bidirectional Power Meter external
            "enabled": true,
            "skip": false,
            "protocol": "sml",
            "device": "/dev/USBserialNetz",
            "baudrate": 9600, 
            "parity": "8n1", 
            "read_timeout": 60, 
            "aggtime": 60,
            "aggmode": "max",
            "aggfixedinterval": true,
            "use_local_time": true,
            "channels" : [
                { 
                    "api": "null", 
                    "uuid": "snip", 
                    "mqtt_topic": "1-0:1.8.0_Bezug",
                    "identifier": "1-0:1.8.0" 
                },
                { 
                    "api": "null", 
                    "uuid": "snip", 
                    "mqtt_topic": "1-0:2.8.0_Einspeisung",
                    "identifier": "1-0:2.8.0" 
                }
            ] 
        },

This addresses issue #420 from 2020.

[r00t-]

thanks, will have a look

[r00t-]

            "channels" : [ { 
                    "api": "null", 

this is #550 :\

[Zugschlus]

            "channels" : [ { 
                    "api": "null", 

this is #550 :\

Yes, I agree. This was just annoying enough for me to do this quick fix instead of refactoring the whole thing.

[r00t-]

your new push contains the commit from #705
but doesn't matter as long as you/we merge that one first