Change topic name "wifi" to "pool/wifi"

[Vrumfondel]

In environments where one MQTT server handles multiple devices it is quite confusing having a topic "wifi" without any information where it belongs to.
So move it under "pool" would be consistent

[jnsbyr]

Your have a point there, but it is not specific for the wifi subtopic. It is relevant for all MQTT brokers that do not auto-prefix the topics with a device id.

If one wants to support multiple identical devices without auto-device-prefix, it is necessary to start the published topics with something like "/intex-sb-h20-AABBCCDDEEFF/wifi/state" where AABBCCDDEEFF could be the WiFi MAC address or some other device unique ID.

As this project is already around for quite a while, this change would break all existing installations. Therefore it must be implemented as option that can be activated via the config file.

This projects accepts merge request, so anyone who is willing can have a go at it. Please remember to describe the feature in the README.