block DNS traffic at the OpenVPN gateway (#4164)

There are many different edge cases where mac and windows clients (and
maybe android too) will revert to using a different DNS server than the
one specified by openvpn.

This is bad news for security reasons. The client is being designed so
it doesn't leak DNS, however we don't want to put all of our eggs in one
basket, so this will block outgoing port 53 (udp and tcp) on the
gateway's firewall from any of the EIP interfaces (thus not blocking DNS
access on the gateway itself).

Change-Id: I84dcfec7fb591cf7e6b356b66b9721feda188177
fix missing semicolon, causing syntax error

Change-Id: Ic7d0f8cc8c0340fdc24cf5ffa4c7018ebac76c7f