RyuApp: allows observers to specify state

This fixes the following exception:

>   $ ryu-manager ryu_app(OF1.2 app)
>   $ sudo mn --controller remote 127.0.0.1
>   $ sudo ovs-vsctl set bridge s1 protocols='[OpenFlow10,OpenFlow12]'
> connected socket:<socket fileno=4 sock=127.0.0.1:6633 peer=127.0.0.1:60456> address:('127.0.0.1', 60456)
> EVENT ofp_event->dpset EventOFPStateChange
> connected socket:<socket fileno=10 sock=127.0.0.1:6633 peer=127.0.0.1:60457> address:('127.0.0.1', 60457)
> EVENT ofp_event->dpset EventOFPStateChange
> hello ev <ryu.controller.ofp_event.EventOFPHello object at 0x11bf550>
> unsupported version 0x1. If possible, set the switch to use one of the versions [3]
> error msg ev version: 0x3 msg_type 0x1 xid 0xc84d9220 type 0x1 code 0x1 0x3 0x1 0x0 0x5f 0xc8 0x4d 0x92 0x20 0x0 0x0 0x0 0x0 0x75 0x6e 0x73 0x75 0x70 0x70 0x6f
> +0x72 0x74 0x65 0x64 0x20 0x76 0x65 0x72 0x73 0x69 0x6f 0x6e 0x20 0x30 0x78 0x31 0x2e 0x20 0x49 0x66 0x20 0x70 0x6f 0x73 0x73 0x69 0x62 0x6c 0x65 0x2c 0x20 0x73
> +0x65 0x74 0x20 0x74 0x68 0x65 0x20 0x73 0x77 0x69 0x74 0x63 0x68
> EVENT ofp_event->dpset EventOFPPortStatus
> DPSET: A port was modified.(datapath id = None, port number = 1)
> Traceback (most recent call last):
>   File "/usr/lib/python2.7/dist-packages/gevent/greenlet.py", line 390, in run
>     result = self._run(*self.args, **self.kwargs)
>   File "/usr/local/lib/python2.7/dist-packages/ryu-1.6-py2.7.egg/ryu/base/app_manager.py", line 86, in _event_loop
>     handler(ev)
>   File "/usr/local/lib/python2.7/dist-packages/ryu-1.6-py2.7.egg/ryu/controller/dpset.py", line 192, in port_status_handler
>     self.port_state[datapath.id].modify(port.port_no, port)
> KeyError: None
> <Greenlet at 0x11bc518: <bound method DPSet._event_loop of <ryu.controller.dpset.DPSet object at 0xd0df10>>> failed with KeyError

Reported-by: HIYAMA Manabu <hiyama.manabu@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>