Add setting of reservations inside of subnets

Host reservations, as noted here: https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#host-reservations-in-dhcpv4 are meant to only contain an IP address if they are specified at the subnet level. Re-use the host reservation mechanism to add reservations in the subnet area. Allow IP address to be set here but only if it is part of the enclosing subnet.