option to override nameserver used in verify_record()

[erachen]

I use successfully dehydrated-hook-ddns-tsig with dns-01 challenge

When the server with dehydrated use a general purpose dns (/etc/resolv.conf) it works.
But, for convenience, I use a local DNS that override public IP of the service with its private IP or simply give a local name to an local (not visible on internet) service.

This allow the server with dehydrated (usually a SSL/TLS http reverse proxy) to use internal name to target local machines, it's more convenient and more meaningful in differents config files.
But if I use an local DNS, for some domain, the DNS check failed.

I think this is related to the verify_record() that use a list of nameserver computed by query_NS_record().
query_NS_record() look for NS record by querying the local resolution config i.e. /etc/resolv.conf.
As it is an internal DNS, NS record may not be consistent with what is expected by let's encrypt (I mean a public view of NS).

So far, I use an external general purpose DNS to solve the problem. It work but I do not have local DNS name anymore.

So I would like to add an optionnal parameter, let say name_server_ip_resolver that, if empty or not specified does not change the behaviour of dehydrated-hook-ddns-tsig.py, but if specified, this IP will be used in query_NS_record().

So that, my general system use local DNS defined in /etc/resolv.conf and if needed, dehydrated-hook-ddns-tsig.py use a different dns for verifying DNS records.