Username not set for mqtt_consumer plugin

Username parameter for the mqtt_consumer plugin was not pass to the client because an incorrect empty check.

closes #781

plugins/inputs/mqtt_consumer/mqtt_consumer.go

@@ -181,7 +181,7 @@ func (m *MQTTConsumer) createOpts() (*mqtt.ClientOptions, error) {
 	}
 
 	user := m.Username
-	if user == "" {
+	if user != "" {
 		opts.SetUsername(user)
 	}
 	password := m.Password