Corresponding nova-api log for delete : 

2016-02-23 10:50:14.473 2792 ERROR nova.network.neutronv2.api [req-8441dee4-9f0c-40a1-84fa-8ade648cafd8 ba0a7bd4bede42eaaf3c4f4f47936da3 6412d041dd49437488453a7df596dff3 - - -] Unable to clear device ID for port 'None'
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api Traceback (most recent call last):
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/nova/network/neutronv2/api.py", line 366, in _unbind_ports
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     port_client.update_port(port_id, port_req_body)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 99, in with_params
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     ret = self.function(instance, *args, **kwargs)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 520, in update_port
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     return self.put(self.port_path % (port), body=body)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 299, in put
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     headers=headers, params=params)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 267, in retry_request
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     headers=headers, params=params)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 208, in do_request
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     self._handle_fault_response(status_code, replybody)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 182, in _handle_fault_response
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     exception_handler_v20(status_code, des_error_body)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api   File "/usr/lib/python2.7/dist-packages/neutronclient/v2_0/client.py", line 80, in exception_handler_v20
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api     message=message)
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api NeutronClientException: 404 Not Found
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api The resource could not be found.
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api
2016-02-23 10:50:14.473 2792 TRACE nova.network.neutronv2.api
2016-02-23 10:50:14.806 2792 INFO nova.compute.manager [req-b4f3e9a6-479a-42b7-b5e8-242fabc959d3 ba0a7bd4bede42eaaf3c4f4f47936da3 6412d041dd49437488453a7df596dff3 - - -] [instance: 3966ce8e-b294-42ba-8b49-e2b0928b9f73] Terminating instance
2016-02-23 10:50:14.811 2792 INFO nova.compute.manager [req-0577d2fb-6c97-4c79-80d8-9e1a1bca0bd4 ba0a7bd4bede42eaaf3c4f4f47936da3 6412d041dd49437488453a7df596dff3 - - -] [instance: 3b840c0c-4c3b-40cc-b48e-b69c16ab9369] Terminating instance
2016-02-23 10:50:15.779 2792 INFO nova.virt.libvirt.driver [-] [instance: 3966ce8e-b294-42ba-8b49-e2b0928b9f73] Instance destroyed successfully.