Add support of DNS server declaration

We need to declare some DNS servers for the virtual machines so they can
resolve host names.

Closes: #386

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
network/dhcp: allow setting & unsetting dnsmasq_dns_server

Also rename dnsmasq_dns_servers to dnsmasq_dns_server as the former is
only supported in Icehouse.

Add unit tests for the feature.