Un-retain old home assistant messages

[lupine]

Reading https://github.com/celsworth/lxp-bridge/wiki/MQTT-Home-Assistant , it became clear to me that the Home Assistant topics have changed name since 0.9, which I hadn't realised before.

In the example, we move from homeassistant/sensor/3333333333_soc/config to homeassistant/sensor/lxp_3333333333/soc/config, for instance. This means controls will be duplicated in HA.

We could advise that people clear out these old topics manually, but we could also manage this inside lxp-bridge by publishing empty-payload messages to the old topics; think of it as a kind of database migration.