solve application dependency via @set_ev_cls

Currently, ryu always starts ofp application because ryu doesn't know
applications that an user specifies needs ofp application or not. So
you can't run two ryu instances even if one of them doesn't use ofp
application because two instances tries to listen the same port.

The root problem is that there is no way to represent dependency
between applications. The patch to invent our own json format to
represent such was proposed ago but I'm still not sure we really need
such complicity.

This tries to solve the problem simply. A module defining events for
an application calls register_service() to declare which application
generates the events, e.g., ofp_event.py call register_service() with
'ofp_handler' argument. If an application starts including @set_ev_cls
for ofp_event, ryu-manager starts ofp application (ofp_handler).

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