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
Merge pull request #65 from SUSE-Cloud/release/pebbles/master

use forward only option
Set "forward only" only if forwarders is not nil
Merge pull request #68 from saschpe/release/pebbles/master-fixup-forward-only-ubuntu

Set forward-only only on SUSE
Merge pull request #69 from crowbar/release/pebbles/master

Sync with pebbles