Add local mDNS responder for .local (#386)

* Add local mDNS responder

* Fix

* Use mDNS in dashboard status

* Lint

* Lint

* Fix test

* Remove hostname

* Fix use enum

* Lint

docker/Dockerfile.hassio

@@ -24,8 +24,6 @@ RUN \
         python-pil \
         # Git for esphomelib downloads
         git \
-        # Ping for dashboard online/offline status
-        iputils-ping \
         # NGINX proxy
         nginx \
     \