Configure the bridge for spanning tree off

Spanning tree causes the bridge port to start in listening mode.
That causes dhcp packet to be ignored. Since we're not fearing of
layer2 loops, we can safely remove the spanning tree config.

Signed-off-by: Dor Laor <dor@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>