physical: Preserve output port across multicast group output.

physical: Preserve output port across multicast group output.

Otherwise actions like this would not output to the same set of ports
for each output action (the second output would only forward to the
last port from the first output action):

    outport = "_MC_FLOOD"; output; output;

Obviously it's a corner case but it still seems worth implementing
correctly.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>