Fixes #2089 - Add network configuration to ENC

I'm not sure I follow. `nil` should never become key, subnets is always array of hashes and some values may be `nil` which becomes `undef` (well I tested only on one puppet version)

I'd say that it would be more convenient to always have `gateway` key which sometimes has address as a value and sometimes is `undef`. Some subnets does not have gateway which is valid value.