NOISSUE - Fix MQTT Forwarder client id (#1309)

* Fix MQTT Forwarder client ID

Don't set client ID for MQTT client in MQTT forwarder. It results in
error in case of multiple instances of the MQTT adapter because they all
share MQTT client ID, which makes broker disconnect all the other
adapters.

Signed-off-by: dusanb94 <dusan.borovcanin@mainflux.com>

* Inline broker address option

Signed-off-by: dusanb94 <dusan.borovcanin@mainflux.com>

* Add MQTT client username

Signed-off-by: dusanb94 <dusan.borovcanin@mainflux.com>

Co-authored-by: Drasko DRASKOVIC <drasko.draskovic@gmail.com>

pkg/messaging/mqtt/pubsub.go

@@ -12,19 +12,16 @@ import (
 
 const (
 	protocol = "mqtt"
-	id       = "mqtt-publisher"
-	qos      = 1
+	username = "mainflux-mqtt"
+	qos      = 2
 )
 
 var errConnect = errors.New("failed to connect to MQTT broker")
 
 func newClient(address string, timeout time.Duration) (mqtt.Client, error) {
 	opts := mqtt.NewClientOptions().
-		AddBroker(address).
-		SetUsername(id).
-		SetPassword(id).
-		SetClientID(id).
-		SetCleanSession(false)
+		SetUsername(username).
+		AddBroker(address)
 	client := mqtt.NewClient(opts)
 	token := client.Connect()
 	if token.Error() != nil {