geoipbackend: accept custom lookup mapping

geoipbackend: accept custom lookup mapping

If for example you want a per country granularity, but still want to
group by custom regions: uk, fr, be -> eu-central, pt, es -> eu-south;
you need to to use %cc and create identical country records. It means
you zones file can become huge and powerdns does not like it too much.

For a user to have a custom mapping without the need to rewrite the
GeoIP database, with this commit, he/she can specify a custom mapping,
so if we get from the GeoIP database fr, we will look in the custom
mapping and find eu-central. So we only need to create the eu-central
record and it will be used by for uk, fr and be as per our first
example.

Definition of mapping_lookup_formats or custom_mapping at the domain
level has priority, but if not defined the global config will be used
as default.

The custom lookup formats and mapping are specified in the zones file:

  ---
  mapping_lookup_formats: ['%cc-%re', '%cc']
  custom_mapping:
    'fr': 'eu-central'
    'be': 'eu-central'
    'es': 'eu-south'
    'pt': 'eu-south'
    'us-ca': 'us-west'
    'us-tx': 'us-south'
  domains:
  - domain: example.com
    services:
      www.example.com: [ '%mp.www.example.com' ]
    records:
      eu-central.www.example.com:
        - A: 1.1.1.1
      eu-south.www.example.com:
        - A: 1.1.1.2
      us-west.www.example.com:
        - A: 1.1.1.3
      us-south.www.example.com:
        - A: 1.1.1.4
  - domain: example2.com
    mapping_lookup_formats: ['%cc']