OVS cleanup utility removes veth pairs

Fixes bug 1094185

OVSInterfaceDriver plug() creates a veth pair and add it to an OVS
bridge when ovs_use_veth=True. It is useful that OVS cleanup utility
removes such veth paris in addition to removing OVS ports.

Veth pairs to be removed are veth devices corresponding to ports
removed when all_ovs_port==False. Veth pairs created to connect
the integration bridge and physical bridges are not removed.

Change-Id: Ia87f0464e69a45dd0bdfd907ac71a003df9fc23f
Merge "OVS cleanup utility removes veth pairs"