when dpid of ovs bridge changed, the flow data of old dpid should be deleted from datastore

step as follows can cause dpid changed of ovs

1. ovs connect to odl using below command
sudo ovs-vsctl set-manager tcp:10.43.174.155:6640
2. creat network, subnet, and vm on openstack.
3 make the ovs reconnect to odl using follow commands
sudo ovs-vsctl set-manager tcp:127.0.0.1:6640
sudo ovs-vsctl set-manager tcp:10.43.174.155:6640

when dpid changed in ovs, the new flow data respond to new dpip is generated(path of NETVIRT-213), but the old flow is not deleted