More support for availability reporting on MQTT components (#11336)

* Abstract MQTT availability from individual components

- Moved availability topic and payloads to MQTT base schema.
- Updated components that already report availability:
  - Switch
  - Binary sensor
  - Cover

* Add availability reporting to additional MQTT components

- Light
- JSON light
- Template light
- Lock
- Fan
- HVAC
- Sensor
- Vacuum
- Alarm control panel

* Annotate MQTT platform coroutines

homeassistant/components/camera/mqtt.py

@@ -60,11 +60,9 @@ class MqttCamera(Camera):
         """Return the name of this camera."""
         return self._name
 
+    @asyncio.coroutine
     def async_added_to_hass(self):
-        """Subscribe MQTT events.
-
-        This method must be run in the event loop and returns a coroutine.
-        """
+        """Subscribe MQTT events."""
         @callback
         def message_received(topic, payload, qos):
             """Handle new MQTT messages."""