Add discovery notify support and mysensors notify (#5219)

* Add mysensors notify platform

* Make add_devices optional in platform callback function.
* Use new argument structure for all existing mysensors platforms.
* Add notify platform.
* Update mysensors gateway.

* Refactor notify setup

* Enable discovery of notify platforms.
* Update and add tests for notify component and some platforms.
* Continue setup of notify platforms if a platform fails setup.
* Remove notify tests that check platform config. These tests are not
  needed when config validation is used.
* Add config validation to APNS notify platform.
* Use discovery to set up mysensors notify platform.

* Add discovery_info to get_service and update tests

* Add discovery_info as keyword argument to the get_service function
  signature and update all notify platforms.
* Update existing notify tests to check config validation using test
  helper.
* Add removed tests back in that checked config in apns, command_line
  and file platforms, but use config validation test helper to verify
  config.
* Add a test for notify file to increase coverage.
* Fix some PEP issues.

* Fix comments and use more constants

* Move apns notify service under notify domain

homeassistant/components/notify/gntp.py

@@ -39,7 +39,7 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
 })
 
 
-def get_service(hass, config):
+def get_service(hass, config, discovery_info=None):
     """Get the GNTP notification service."""
     if config.get(CONF_APP_ICON) is None:
         icon_file = os.path.join(os.path.dirname(__file__), "..", "frontend",