Support local (10.x, 172.x, 192.168.x) reverse lookups in cache

By default djbns::cache will leak reverse lookups for 10.x, 172.x, 192.168.x. djbdns::internal_server will respond to the reverse lookups correctly, but the cache only refers to the internal server for {{node['djbdns']['tinydns_internal_resolved_domain']}}, not 168.192.in-addr.arpa or similar.

A super fancy technique could look at the ipaddresses and netmasks to generate the right reverse lookup domain, but it's probably easier (and less surprising) to have it be an explicit attribute.