isc-dhcp: treat 'config host' like superset of 'config domain'
Not including an A record mapping will cause nsupdate to balk at
CNAME and MX records (and probably SRV as well) because the target
will be unknown at the time of parsing, until the lease gets
activated.
We need these RR's to be in place well before the servers even
come up.
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>