Allow multiple dns servers when starting dnsmasq

Previously we only allowed one upstream dns server even though we
can store two in the network object. This changes the dns_server
flag to a ListOpt so it can be specified multiple times. It also
uses the dns servers from the network if they are set. Previously
these dns servers were simply ignored.

DocImpact

Change-Id: I4c96259bce25036392afe6e027ec47b92679ecbc