refactor(mqtt): revert mqtt source to use /etc/mqtt_source.yaml as the config file

docs/en_US/rules/sources/mqtt.md

@@ -1,6 +1,6 @@
 # MQTT source 
 
-Kuiper provides built-in support for MQTT source stream, which can subscribe the message from MQTT broker and feed into the Kuiper processing pipeline.  The configuration file of MQTT source is at ``$kuiper/etc/sources/mqtt.yaml``. Below is the file format.
+Kuiper provides built-in support for MQTT source stream, which can subscribe the message from MQTT broker and feed into the Kuiper processing pipeline.  The configuration file of MQTT source is at ``$kuiper/etc/mqtt_source.yaml``. Below is the file format.
 
 ```yaml
 #Global MQTT configurations

xstream/nodes/source_node.go

@@ -74,7 +74,11 @@ func getConf(t string, confkey string, ctx api.StreamContext) map[string]interfa
 	if t == ""{
 		t = "mqtt"
 	}
-	conf, err := common.LoadConf("sources/" + t + ".yaml")
+	confPath := "sources/" + t + ".yaml"
+	if t == "mqtt"{
+		confPath = "mqtt_source.yaml"
+	}
+	conf, err := common.LoadConf(confPath)
 	props := make(map[string]interface{})
 	if err == nil {
 		cfg := make(map[string]map[string]interface{})