openvswitch: Use tun_key only for egress tunnel path.

Currently tun_key is used for passing tunnel information
on ingress and egress path, this cause confusion.  Following
patch removes its use on ingress path make it egress only parameter.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>