Omit mode keyword when spawning dnsmasq with some ipv6 subnets

Fix UnboundLocalError for certain cases of ipv6 subnets

Closes-Bug: #1335959
Change-Id: I5e4b94d9ba1b13b72e2300b53d5638968e0d3401
Merge "Omit mode keyword when spawning dnsmasq with some ipv6 subnets"