Fix plugin short name parsing for records that contain the zone name more than once

[rmbolger]

Fixes #584

Many DNS plugins use a regex based algorithm documented in the plugin development guide to separate a record label (short name) from its zone name. The previous version of this algorithm didn't account for the edge case where the FQDN might contain the zone name more than once in the string.

So a record like _acme-challenge.example.com.foo.example.com in a zone called example.com would result in a short name of _acme-challenge..foo instead of _acme-challenge.example.com.foo preventing the proper creation of the associated TXT record with that provider.

This change fixes that bug in all of the plugins currently implementing it and in the plugin development guide.

Thanks to @andreashaerter for finding the original bug.