Add output type to mqtt node

Fixes #1912

nodes/core/locales/en-US/messages.json

@@ -324,6 +324,7 @@
         "label": {
             "broker": "Server",
             "example": "e.g. localhost",
+            "output": "Output",
             "qos": "QoS",
             "retain": "Retain",
             "clientid": "Client ID",
@@ -358,6 +359,10 @@
             "connect-failed": "Connection failed to broker: __broker__"
         },
         "retain": "Retain",
+        "output": {
+            "buffer": "a Buffer",
+            "string": "a String"
+        },
         "true": "true",
         "false": "false",
         "tip": "Tip: Leave topic, qos or retain blank if you want to set them via msg properties.",