Adding back single flood flow.
Openflow specification confirms that oputput action would not
send a packet out of ingress port unless reserved port is used.
So it is safe to use single flood flow that only discards 'discarding'
ports.

Change-Id: Ic82013ad0ea3915b217884582c0fd9552bd21c7c
Signed-off-by: Amit Mandke <ammandke@cisco.com>