ofproto: exception during startup related ofp module detaction

This patch fixes the following exception.

ryu/controller/ofp_event.py: update
cd /opt/stack/ryu && /opt/stack/ryu/bin/ryu-mana ^Mger --config-file /etc/ryu/ryu.conf || touch "/opt/stack/status/stack/ryu.failur ^Me"
> Traceback (most recent call last):
>   File "/opt/stack/ryu/bin/ryu-manager", line 41, in <module>
>     from ryu.base.app_manager import AppManager
>   File "/opt/stack/ryu/ryu/base/app_manager.py", line 22, in <module>
>     from ryu.controller.handler import register_instance
>   File "/opt/stack/ryu/ryu/controller/handler.py", line 20, in <module>
>     from ryu.controller import ofp_event
>   File "/opt/stack/ryu/ryu/controller/ofp_event.py", line 69, in <module>
>     for ofp_mods in ofproto.get_ofp_module():
> TypeError: get_ofp_module() takes exactly 1 argument (0 given)

Reported-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
tests/ofproto: add unit test for ofproto automatic detection

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>