rstp: Show some useful rstp port fields.

designated_bridge_id, designated_port_id and designated_path_cost are
now displayed in rstp_status when using 'ovs-vsctl list port'.

Signed-off-by: Daniele Venturino <daniele.venturino@m3s.it>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>