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
Skip extra logging when devices is empty

Change-Id: Id9ae8aef0555109bd88600965adb2744ab924430
Closes-Bug: #1278271
Merge "Allow multiple DNS forwarders for dnsmasq"
Merge "Skip extra logging when devices is empty"