take 3 on protocol config: use enum in config schema to enforce allowed values

keep `MQTTv311` default in sensor class in case `protocol` is not specified in pack config

Author: smuskiew

Diff for: config.schema.yaml

@@ -18,6 +18,11 @@
   protocol:
     description: "MQTT protocol version. Default MQTTv311"
     type: "string"
+    enum:
+    - MQTTv3
+    - MQTTv311
+    - MQTTv5
+    default: MQTTv311
     required: false
     secret: false
   client_id:

Diff for: sensors/mqtt_sensor.py

@@ -22,10 +22,7 @@ def __init__(self, sensor_service, config=None):
         self._client = None
         self._hostname = self._config.get('hostname', None)
         self._port = self._config.get('port', 1883)
-        # NOTE: Need to translate MQTT* names into values expected by paho
-        # Default to MQTTv311 if value is not valid since we can't
-        # enforce "allowed" values from the pack config schema
-        self._protocol = getattr(mqtt, self._config.get('protocol', ''), mqtt.MQTTv311)
+        self._protocol = self._config.get('protocol', 'MQTTv311')
         self._client_id = self._config.get('client_id', None)
         self._userdata = self._config.get('userdata', None)
         self._username = self._config.get('username', None)
@@ -39,8 +36,11 @@ def __init__(self, sensor_service, config=None):
     def setup(self):
         self._logger.debug('[MQTTSensor]: setting up sensor...')
 
+        # NOTE: Need to ensure `protocol` MQTT* names are properly
+        # handled as paho.mqtt constants and not bare strings
         self._client = mqtt.Client(self._client_id, clean_session=True,
-                             userdata=self._userdata, protocol=self._protocol)
+                                   userdata=self._userdata,
+                                   protocol=getattr(mqtt, self._protocol))
 
         if self._username:
             self._client.username_pw_set(self._username, password=self._password)