Close an open connection to an MQTT broker
Optional function called upon disconnection
This method closes the mqttclient’s connection to its specified MQTT broker.
Optionally, you may provide a function which will be called when the connection has been closed. The callback function should have no parameters of its own.
Before calling mqttclient.disconnect(), you can use the imp API method mqttclient.isconnected() to determine whether the client is connected or not.