ovn-northd: no logical router icmp response for directed broadcasts

ovn-northd: no logical router icmp response for directed broadcasts

Responding to icmp queries where the L3 destination is a directed broadcast
was not being properly handled, causing the reply to be sent to all logical
ports except for the one port that should receive it.

This is a proposal for using choice B in the mail discussion; where icmp
queries to broadcast are simply not responded by the logical router.

Reported-at: http://openvswitch.org/pipermail/discuss/2016-June/021610.html
Signed-off-by: Flavio Fernandes <flavio@flaviof.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>