[certbot-dns-google] Lookup zone based on validation name instead of domain

[jonas-p]

If you've set up delegation of the _acme-challenge subdomain to a validation-specific zone in Google Cloud DNS the plugin fails to find the correct zone since the current lookup is based on the domain.

E.g.
Lets say I have two zones in two separate GCP projects (Cloud DNS):

• Zone 1 (project-a): example.com with a NS record that delegates _acme-challenge.example.com to Zone 2.
• Zone 2 (project-b): _acme-challenge.example.com.

Running certbot with --dns-google-project project-b --domain example.com will fail to find the zone in project b.

This changes the zone lookup for the Google Cloud DNS validator to the validation_name instead of the domain argument. The validation_name is usually just the domain name with the "_acme-challenge." prefix.

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