clientID removed in the MQTT config (#7157)

More than one client can't use the same clientID for MQTT connection. 
This causes problem in distributed deployments where config is shared 
across nodes, as each Minio instance tries to connect to MQTT using the
same clientID.

This commit removes the clientID field in config, and allows
MQTT client to create random clientID for each node.

pkg/event/target/mqtt.go

@@ -36,7 +36,6 @@ type MQTTArgs struct {
 	Broker               xnet.URL       `json:"broker"`
 	Topic                string         `json:"topic"`
 	QoS                  byte           `json:"qos"`
-	ClientID             string         `json:"clientId"`
 	User                 string         `json:"username"`
 	Password             string         `json:"password"`
 	MaxReconnectInterval time.Duration  `json:"reconnectInterval"`
@@ -59,9 +58,15 @@ func (m MQTTArgs) Validate() error {
 	default:
 		return errors.New("unknown protocol in broker address")
 	}
-	if m.QueueDir != "" && !filepath.IsAbs(m.QueueDir) {
-		return errors.New("queueDir path should be absolute")
+	if m.QueueDir != "" {
+		if !filepath.IsAbs(m.QueueDir) {
+			return errors.New("queueDir path should be absolute")
+		}
+		if m.QoS == 0 {
+			return errors.New("qos should be set to 1 or 2 if queueDir is set")
+		}
 	}
+
 	return nil
 }
 
@@ -120,7 +125,7 @@ func (target *MQTTTarget) Close() error {
 // NewMQTTTarget - creates new MQTT target.
 func NewMQTTTarget(id string, args MQTTArgs) (*MQTTTarget, error) {
 	options := mqtt.NewClientOptions().
-		SetClientID(args.ClientID).
+		SetClientID("").
 		SetCleanSession(true).
 		SetUsername(args.User).
 		SetPassword(args.Password).