Fixes race condition of deleting floating ip

Fixes bug #1172599
reproduce steps:
1. create an instance
2. get a floating ip from pool
3. associate the ip to instance created at step 1
4. delete the instance and the ip at a very little interval

reason may be that:
1. the action of delete ip find the ip is associated to a fixed ip at
   first(at api level)
2. and then the fixed ip is released when the instance is deleted
3. so nova-network manager findes this issue and raises
   exception.FloatingIpNotAssociated(address=floating_address)

Change-Id: I253d0b240451e8a728532639e9857894f6fc6870
Merge "Fixes race condition of deleting floating ip"