Add OpenTherm Gateway climate platform (#16299)

* Initial OpenTherm Gateway support.

* Fix coveragerc and requirements_all*.txt
Overall cleanup and polishing

* Make hound/flake/travis happy

* Basic improvements to comply with Home Assistant's style guidelines
Changed wording from "component" to more appropriate terms where necessary
Fixed small mistakes that snuck in during testing and/or due to my own ignorance ;)

* Fixed overwriting state property

* Fixed a bug with ROOM_SETPOINT_OVRD
Updated dependency pyotgw to latest version

* Remove unit_of_measurement from OpenThermGateway class

* Cleanup after previous commits

* Moved initialisation and configuration from async_setup_platform to async_added_to_hass

* Make travis happy

* Disable polling for this platform
Improve update flow

* Small improvements/optimisations