Fix MQTT retained message not being re-dispatched (#12004)

* Fix MQTT retained message not being re-dispatched

* Fix tests

* Use paho-mqtt for retained messages

* Improve code style

* Store list of subscribers

* Fix lint error

* Adhere to Home Assistant's logging standard

"Try to avoid brackets and additional quotes around the output to make it easier for users to parse the log."
 - https://home-assistant.io/developers/development_guidelines/

* Add reconnect tests

* Fix lint error

* Introduce Subscription

Tests still need to be updated

* Use namedtuple for MQTT messages

... And fix issues

Accessing the config manually at runtime isn't ideal

* Fix MQTT __init__.py tests

* Updated usage of Mocks
* Moved tests that were testing subscriptions out of the MQTTComponent test, because of how mock.patch was used
* Adjusted the remaining tests for the MQTT clients new behavior - e.g. self.progress was removed
* Updated the async_fire_mqtt_message helper

* ✅ Update MQTT tests

* Re-introduce the MQTT subscriptions through the dispatcher for tests - quite ugly though...  🚧
* Update fixtures to use our new MQTT mock 🎨

* 📝 Update base code according to comments

* 🔨 Adjust MQTT test base

* 🔨 Update other MQTT tests

* 🍎 Fix carriage return in source files

Apparently test_mqtt_json.py and test_mqtt_template.py were written on Windows. In order to not mess up the diff, I'll just redo the carriage return.

* 🎨 Remove unused import

* 📝 Remove fire_mqtt_client_message

* 🐛 Fix using python 3.6 method

What's very interesting is that 3.4 didn't fail on travis...

* 🐛 Fix using assert directly

tests/conftest.py

@@ -8,11 +8,11 @@ from unittest.mock import patch, MagicMock
 import pytest
 import requests_mock as _requests_mock
 
-from homeassistant import util, setup
+from homeassistant import util
 from homeassistant.util import location
-from homeassistant.components import mqtt
 
-from tests.common import async_test_home_assistant, mock_coro, INSTANCES
+from tests.common import async_test_home_assistant, INSTANCES, \
+    async_mock_mqtt_component
 from tests.test_util.aiohttp import mock_aiohttp_client
 from tests.mock.zwave import MockNetwork, MockOption
 
@@ -85,17 +85,9 @@ def aioclient_mock():
 @pytest.fixture
 def mqtt_mock(loop, hass):
     """Fixture to mock MQTT."""
-    with patch('homeassistant.components.mqtt.MQTT') as mock_mqtt:
-        mock_mqtt().async_connect.return_value = mock_coro(True)
-        assert loop.run_until_complete(setup.async_setup_component(
-            hass, mqtt.DOMAIN, {
-                mqtt.DOMAIN: {
-                    mqtt.CONF_BROKER: 'mock-broker',
-                }
-            }))
-        client = mock_mqtt()
-        client.reset_mock()
-        return client
+    client = loop.run_until_complete(async_mock_mqtt_component(hass))
+    client.reset_mock()
+    return client
 
 
 @pytest.fixture