Multiple Slave Support - Object ID setting

[georgegougoudis]

Implement set_object_id.
As per MQTT Naming

If the object_id option is set, then this will be used to generate the entity_id. If, for example, we have configured a sensor, and we have set object_id to test, then Home Assistant will try to assign sensor.test as entity_id, but if this entity_id already exits it will append it with a suffix to make it unique, for example, sensor.test_2.

uniq_id is used only to ensure uniqueness but if the sensor already exists on the master device is it named, as the decumentation mentions, for example sensor.test_2. By providing the object as (unique_prefix)_(entity_name) the sensor is automatically named to something more intuitive.