cmqttd: Only show lighting group addresses which actually exist

[Nik19792000]

First of all, this project is fantastic. It was super easy to link my C-Bus to Home-Assistant. The only slightly annoying part is, that there are automatically 256 devices and more than 500 entities, even though my C-Bus only has around 10 light groups. I understand the reason that C-Bus can theoretically address 256 items and this will allow to control all of them, but I have the following feature request:

1. Discover how many devices/groups are actually in use and only make the same number discoverable in MQTT.
2. If the first one is not possible, then add a docker start parameter, which allows to limit the number of devices.

[micolous]

Yup, I agree, it's on a TODO list. 😄

The other way around this is to use the CBZ to only report status for devices that are mentioned there; but this will force you to have a Toolkit Project.

On the C-Bus side, there's some group address status broadcast, but I'm unsure how reliable this is. I started implementing support for this a while ago. I think this only works when a load (ie: dimmer/relay) is assigned to a group address (so no virtual addresses).

[beerygaz]

This would be a big help when synchronising from HA to HomeKit too, as Homekit only supports 150 entities per bridge.