allow RyuApp start() method returns Greenlet thread

This enables an application to continue. Without this, ryu-manager
finishes right after running applications.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ofp: use RyuApp start() to return Greenlet thread

starts OFP like other applications. Kill the original way to start OFP
(start_service).

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