Use 404 instead of 400 when security_group is non-existed

This patch correct the status code when security_group is
non-existed. This is good for sharing unittest between v2.1
and v2 API.

Change-Id: Ib4bd725381052947859db1ab30905fa33e502679
Merge "Use 404 instead of 400 when security_group is non-existed"