ovs-router: Add "ovs/route/lookup" command

This command is useful at least for testing.

Example output:
    % ovs-appctl ovs/route/lookup '10.0.0.1'
    gateway 172.17.0.254
    dev wm0
    %

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Pravin B Shelar <pshelar@nicira.com>