Allow to disable default gw feature

Allow to disable default gw feature

- Default gateway is a good feature for having host discovery, and
  consequently hosts' pings, work out of the box.
  But in real scenarios a controlled network node may be connected to a
  legacy switch's trunk port which exposes thousand of hosts which ARP
  packets would easily stress controller functionality.
  Even though today this can be prevented by configuring a more
  specific subnet gateway, the issue is very much possible at first
  controller boot, and can always re-occur during subnet config manipulation
  if at any point of time we have no user configured subnet gateways.
- This patch provides a way to disable he default gw feature by mean of
  a property in config.ini. Property addition is not included, current
  behavior is unchanged.

Change-Id: I15b63233e3ed46a8a9fb57bb0988a0d4a2984b11
Signed-off-by: Alessandro Boch <aboch@cisco.com>