ryu.conf: obsoleted argument used in sample configuration

ofp_listen_port was obsoleted and users should use ofp_tcp_listen_port
(or ofp_ssl_listen_port if ssl enabled) instead.

Signed-off-by: Wei-Li Tang <alextwl@xinguard.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
add ryu.cfg

also, make CONF accessible via RyuApp.CONF.
add a comment to explain the intention.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.cfg: re-export some frequently used stuff for convenience

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
reduce direct uses of oslo.config.cfg.CONF

no functional changes are intended.

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