dhcp: T6948: systemd-service adjustment to invoke helper script

The helper script updates VyOS hostd records from
DHCP server leases. This ensures that hostd records
with the DHCP server leases are kept in sync with
VyOS hostd records right after the DHCP server is
started.

Note that Restart directive needs to be updated to
on-failure so that the service is restarted in case
of failure/timeout in interaction with hostd service.