Disable intermediary bridge when run on OVS conntrack enabled kernels

[awh]

#1577 will introduce an intermediary bridge netdev because the kernels in current distributions do not support connection tracking in conjunction with the OVS datapath. We believe that this functionality has been added in the 4.3 kernel - we need to verify whether it meets Scope's needs, and then implement automatic detection that bypasses the intermediary bridge to avoid its performance cost.