ETCD TLS support

[AlessandroZanatta]

What would you like to be added: I'd like to use an existing etcd cluster in order to have external-dns work with coredns

Why is this needed: Allows re-using an existing etcd cluster instead of creating a new one. Helps in more resource-constrained environments too, albeit its possible security downsides.

I could send a PR in the next week (hopefully) if this feature is welcome. Could an implementation along the lines of #4503 be accepted? I think the implementation really just needs to pass along some config variables to golang's etcd client.

[AlessandroZanatta]

Looking around in the code I see that it has already been implemented, but I think the documentation is missing. I will be sending a PR to fix that soon.