ArpHandler to ignore ip packet sent to default GW

ArpHandler to ignore ip packet sent to default GW

- If the host pings the default gw, when ArpHandler receives the
  ICMP packet it does not realize it is sent to the controller,
  and it triggers the discovery for an host with that IP through arp request
- This patch is for ArpHandler to detect the if IP packet is sent
  to the controller and stop the processing if that is the case.
  We may consider to have ArpHandler reply if packet is ICMP echo req.

Change-Id: I87e14916564bc0039718882d087c431df7554317
Signed-off-by: Alessandro Boch <aboch@cisco.com>