network/dhcp: Update deprecated option dnsmasq_server

dnsmasq_dns_server is deprecated since approx. 1 year,
the new option is dnsmasq_dns_servers (an array of DNS IP).

See:
https://github.com/openstack/neutron/commit/a269541c603f8923b35b7e722f1b8c0ebd42c95a

Change-Id: I752fc0c6b176d36e9fc5d35609e533f2659afdd4
Merge "network/dhcp: Update deprecated option dnsmasq_server"