Gracefully handle duplicate delete requests.
Gracefully handle duplicate delete requests.

This code change is to handle any gracefully handle duplicate delete requests
sent by external entities, in this case service monitor.
This code change will log and handle the key not found excpetion and proceed.

The service monitor, sending  duplicate deletes is tracked by bug: 1636932

Change-Id: I606561ca0e3c570713b41c196325180a16f96987
closes-bug: #1635901

rombie/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller