If VN IPAM has a gateway of 0.0.0.0 or from link local range (169.254.0.0/16) then mark VN's
forwarding mode to be L2 only.

https://bugs.launchpad.net/juniperopenstack/+bug/1370328

Change-Id: I056eb2105c37c6f2057401e2d428a4ceec7cede3