bind - optionally add static dhcp leases at zone A records

Similar to 'unbound' feature to avoid the need to duplicate static host
addresses in both DHCP and BIND.

- Add 'Register DHCP static mappings' checkbox to zone form
- if checked, enumerate all static DHCP hosts and add to zone, such that
their name can be resolved.
Disable logging should not mean default logging

- Prevent bind from flooding logs when 'enable logging' is not checked.
Merge pull request #536 from jean-m-cyr/master

bind - optionally add static dhcp leases as zone A records