Add lock on API layer delete floating IP

There are potential race condition when multiple nova-api
service is running. We need to prevent the concurrent access to
the same floating ip, though db layer function is ok for floating
ip, the quota will be released more than one time and it will
lead to potential problem later

So the solution: check whether the db is *really* updated with the
value we want, which means the second operation will fail and
return None, so network api layer can handle it

Change-Id: I028938a4c387d21a8880ae168b161a763e7361ff
Closes-Bug: #1268569
Merge "Add lock on API layer delete floating IP"