config: Change MQTT client ID

Since the application can act both as publisher and subscriber
use more generic name - "client".

Signed-off-by: Zbigniew Bodek <zbigniew.bodek@huawei.com>

src/config.h

@@ -39,6 +39,6 @@
 
 #define APP_MQTT_BUFFER_SIZE	128
 
-#define MQTT_CLIENTID		"zephyr_publisher"
+#define MQTT_CLIENTID		"zephyr_mqtt_client"
 
 #endif