Automatically detect domains

[Stefan H.]

Description of the feature
As I have a lot of domains pointing to my traefik instance, it would be nice if I wouldn't had to add them manually as environment variables.
The feature would basically look up if the cloudflare account has a zone for each of the used hosts and use it, and if it doesn't, just ignore the entry and throw a warning.

Benftits of feature
You would not have to add all domains manually to the config, which can be messy with a lot of domains, especially as this companion is designed to make things easier with cloudflare

Thanks a lot, keep up the great work!

[modem7]

This could be easily grabbed via entrypoints.https.http.tls.domains