Allow multiple DNS forwarders for dnsmasq

This patch change the dnsmasq_server configuration option to a ListOpt
in order to enable user to specify multiple DNS forwarders for each
dnsmasq instance.

DocImpact

Change-Id: I21963b4a6c99e4edb11040d77a6aeaa35ff44641
Closes-bug: #1240027
Merge "Allow multiple DNS forwarders for dnsmasq"