quantum_adapter: race between ovs port deletion and plugin port deletion

The order between the notification of ovs port deletion via OVSDB protocol and
the notification network id/port deletion via REST from quantum plugin
isn't deterministic.
So when ovs port is deleted, the corresponding network id may or may not
exist.

The code wrongly assumed the order, so resulted in the following exception.
When ovs port is deleted and the corresponding network id isn't find,
just ignore the exception.

> (19257) accepted ('127.0.0.1', 36841)
> 127.0.0.1 - - [19/Jun/2013 11:24:25] "DELETE /v1.0/tunnels/networks/8179bb70-a63f-4c74-a82e-a21f3c275c9a/key HTTP/1.1" 200 115 0.000383
> hub: uncaught exception: Traceback (most recent call last):
>   File "/opt/stack/ryu/ryu/lib/hub.py", line 50, in _launch
>     func(*args, **kwargs)
>   File "/opt/stack/ryu/ryu/base/app_manager.py", line 104, in _event_loop
>     handler(ev)
>   File "/opt/stack/ryu/ryu/app/quantum_adapter.py", line 368, in
> port_del_handler
>     self._port_handler(ev.dp.id, port.port_no, name, False)
>   File "/opt/stack/ryu/ryu/app/quantum_adapter.py", line 336, in _port_handler
>     ovs_switch.update_port(port_no, port_name, add)
>   File "/opt/stack/ryu/ryu/app/quantum_adapter.py", line 279, in update_port
>     self._update_vif_port(old_port, add=False)
>   File "/opt/stack/ryu/ryu/app/quantum_adapter.py", line 194, in
> _update_vif_port
>     self.network_api.remove_port(network_id, self.dpid, port.ofport)
>   File "/opt/stack/ryu/ryu/controller/network.py", line 368, in remove_port
>     self.networks.remove(network_id, dpid, port_no)
>   File "/opt/stack/ryu/ryu/controller/network.py", line 119, in remove
>     raise NetworkNotFound(network_id=network_id)
> NetworkNotFound: no such network id 8179bb70-a63f-4c74-a82e-a21f3c275c9a

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
---
Changes v1 -> v2:
- typo
  self.log -> self.logger
app/quantum_adapter: pylint

W: 18,0: Unused import logging
W: 28,0: Unused import event

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
controller/network: pylint

W: 18,0: Unused import logging

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>