Multiple device support for a single plant

Currently a plant is associated to a single Smartbox device in Telegea.

It should be possible to handle multiple devices for one plant. This could be e.g. a private home or office building with a Smartbox for each floor. The user needs to be able to handle the sensors and actuators connected to these different devices from the same plant user interface on the Telegea server.

To achieve this we might introduce the concept of "device Id", to be added to the existing "plant Id". The user does not necessarily need to know about the different devices, it can be hidden. But the Telegea server needs to know which device is sending certain sensor data or which device to send a certain command to.