base/app_manager.py: sort out registering bricks

When registering RyuApps to observers with _EVENTS,
it should applied to @set_ev_cls methods, and dispatchers should be
honored.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu/base/app_manager: introduce RyuApp:replay_to_request()

This is a handy method for event request/reply.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu/topology/switches: simplify Switches with RyuApp.reply_to_request

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