This doesn't make sense as a port can't be listened to twice on the same domain, so instead we should just fail immediately if someone attempts to perform this action.
Refs dokku/dokku-letsencrypt#269
This change makes interacting with port mappings more clear - folks might previously set the port mapping to the proxy type or vice-versa.
The existing proxy:ports* commands still exist but will show a deprecation warning for a single minor release.
