Add option to use different domain on Gandi then requested for the certificate

[gr0ebi]

Hi!

I have the following setup:

main.tld without DNS API access on a random domain provider
other.tld at Gandi with Live DNS API
I have CNAMEs pointing from main.tld to other.tld (also on all subdomains)

In order to manually update my main.tld certs, I change the TXT values on my other.tld entries to the TXT values from certbot's main.tld responses
The use of a "throwaway" domain for validation is described in the DNS-01 challenge docs (https://letsencrypt.org/docs/challenge-types/)

So my current issue with the plugin is that I need to request main.tld certs but certbot-plugin-gandi should use a different domain (on Gandi DNS) to make my main.tld CNAME entries work

It would be nice to have a parameter in the gandi.ini to provide a "custom" domain name that the plugin should use instead of the domain that was given with certbots "-d" parameter

something like:

# Gandi personal access token
dns_gandi_token=PERSONAL_ACCESS_TOKEN
dns_gandi_verify_domain=other.tld