Prevent DNS server conflicts in CreateBridgeIface

Retrieve /etc/resolv.conf data (if available)

Add checkNameserverOverlaps and call it to
make sure there are no conflicts

Add utils.GetNameserversAsCIDR and tests

Read /etc/resolv.conf and pull out nameservers,
formatting them as a CIDR block ("1.2.3.4/32")
Merge pull request #1577 from mrallen1/fix/1502

Prevent DNS server conflicts in CreateBridgeIface