Support for N-DJBDNS

Add support for N-DJBDNS for cobbler (fixes #1158).

Still to do:

- [ ] Management of SOA and NS records (can currently be done statically in the template)
- [ ] Make data file location configurable (currently hardcoded to `/etc/ndjbdns/data`)
- [ ] Determine `tinydns-data` location dynamically (currently hardcoded to `/usr/bin/tinydns-data`)

Please note that these changes are largely untested. I'm still struggling to build an RPM package from the current master branch.