dont create invalid bind config

when one IP appeared twice in hosts list
two identical entries were added to the reverse DNS zone
which broke bind reload and thus chef
Merge pull request #58 from SUSE-Cloud/release/pebbles/master

https://github.com/crowbar/barclamp-dns/pull/58

Merged by devtool for ArkadyKanevsky
use forward only option

- this is needed to resolve reverse DNS for external private IPs
  because bind 9.9 defaults to empty-zones-enable yes
- this allows to better run behind firewalls