Always update sensor_name after resolving hostname

[kroecks]

My pi's weren't registering correctly and I was getting a lot of MQTT traffic with rpi_{hostname}.
This is because the "default_sensor_name" is not the same as the MQTT.sensor_name config, and yet the default sensor name is being used to validate whether to set the hostname.

Until the logic for reconciling the MQTT config and the default sensor name is more meaningful, it's better to just ALWAYS set the sensor name correctly.

[ironsheep]

Thank you, Ken, for spotting this. This issue has been plaguing many of us. I appreciate your taking the time to file this PR! -Stephen