Properly blackhole IPv6 traffic in GCE + OpenVPN setup.

This fixes two issues:
1. Custom built dnsmasq did not overwrite the original dnsmasq.
   As a result, AAAA DNS lookups returned non-empty results letting
   clients resolve DNS names to IPv6 addresses.
2. OpenVPN server wasn't assigned an IPv6 address and thus didn't
   correctly push IPv6 settings to clients.

Fixes #28
Merge pull request #29 from google/fix-gce-openvpn-ipv6

Properly blackhole IPv6 traffic in GCE + OpenVPN setup.