Catch up with the upstream change in dhcp driver

A change in upstream neutron:
https://github.com/openstack/neutron/commit/9569b2fe58d0e836071992f545886ca985d5ace8

changed dhcp driver interface.
This patch quickly fixes the issue just by making use of
Dnsmasq driver's methods.

Fixes: MN-2757

Change-Id: I4c8a4476ed82e4a4efdd762ae4f14587871a02a6