DHCPv4 server must interpret unspecified subnet and shared network parameters fetched from the DB

The DHCPv4 server fetches the subnets and shared networks from the database and merges them into the current configuration. For all subnet and network specific values that are marked "unspecified" it must use the higher level values, e.g. globals. This work depends on #487 (closed). For details see: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#encoding-other-unspecified-parameters