MQTT discovery (#5724)

* Change implementation

* Re-write

* Remove unused consts

* Update discovery.py

* Add tests

* fix other tests

* Fix check_config script test

* Lint

* Lint

tests/conftest.py

@@ -1,12 +1,14 @@
 """Setup some common test helper things."""
 import functools
 import logging
+from unittest.mock import patch
 
 import pytest
 import requests_mock as _requests_mock
 
-from homeassistant import util
+from homeassistant import util, bootstrap
 from homeassistant.util import location
+from homeassistant.components import mqtt
 
 from .common import async_test_home_assistant
 from .test_util.aiohttp import mock_aiohttp_client
@@ -58,3 +60,18 @@ def aioclient_mock():
     """Fixture to mock aioclient calls."""
     with mock_aiohttp_client() as mock_session:
         yield mock_session
+
+
+@pytest.fixture
+def mqtt_mock(loop, hass):
+    """Fixture to mock MQTT."""
+    with patch('homeassistant.components.mqtt.MQTT') as mock_mqtt:
+        loop.run_until_complete(bootstrap.async_setup_component(
+            hass, mqtt.DOMAIN, {
+                mqtt.DOMAIN: {
+                    mqtt.CONF_BROKER: 'mock-broker',
+                }
+            }))
+        client = mock_mqtt()
+        client.reset_mock()
+        return client