Custom MQTT clientID

[renebohne]

1. Are you using Helium hosted Console or running your own open source Console? Helium hosted Console, but this is a general request.

Please describe your issue: Currently we use the Helium DeviceID as the clientID for mqtt connections. I need to use my own custom mqtt clientID. Solution: add an MQTT clientID field to the MQTT integration and use the helium DeviceID as the default value, but let the user enter its own MQTT clientID.