Fix race condition in api server while deleting multiple objects

Consider the case where the last ip address in a subnet is deleted by api server 1.
Immediately after that, the network to which it belongs being deleted by api server 2.
Then, if api server 2 receives notification for ip address being deleted, it won't find the network in subnet_objs.
We need to ignore this error.

Change-Id: I31ac2280967cda63d8428e24bec65e0e2e46dcbb
Closes-Bug: 1377994
Merge "Fix race condition in api server while deleting multiple objects"