Domain name lower case enforcement

[trinopoty]

This is related to #6972 on Certbot. Lexicon uses == for comparison of domain names. Since python's == is case sensitive for strings and domain names are case insensitive, this causes issues sometimes as in the issue mentioned above.

This should be fixed so that all operations on domain names first convert to lower case.

[marcelloromani]

Shouldn't all DNS names be forced lowercase? (I mean converted to lowercase upon input)

[trinopoty]

It should be but there are inconsistencies in various places. For instance, the linode api does not convert domain names to lower case by default. This caused issues with certbot that converts all domain name to lower case.

This issue is a call to check all provider code to make sure they don't treat domain names anywhere as case sensitive.