gre_tunnel.py: race condition

When tunnel key add/del event fires, the corresponding network_id
does not necessarily exists due to race condition.
So mask it.

(15612) accepted ('127.0.0.1', 45714)
127.0.0.1 - - [25/Jun/2013 12:18:18] "DELETE
/v1.0/networks/462be334-3160-48ae-9ef8-a456dc866f8f HTTP/1.1" 200 115
0.000393
(15612) accepted ('127.0.0.1', 45715)
127.0.0.1 - - [25/Jun/2013 12:18:18] "DELETE
/v1.0/tunnels/networks/462be334-3160-48ae-9ef8-a456dc866f8f/key
HTTP/1.1" 200 115 0.000260
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/gre_tunnel.py", line 233, in
tunnel_key_del_handler
    for (dpid, port_no) in self.nw.list_ports(network_id):
  File "/opt/stack/ryu/ryu/controller/network.py", line 325, in list_ports
    return self.networks.list_ports(network_id)
  File "/opt/stack/ryu/ryu/controller/network.py", line 103, in list_ports
    raise NetworkNotFound(network_id=network_id)
NetworkNotFound: no such network id 462be334-3160-48ae-9ef8-a456dc866f8f

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