add L2populationRpcCallBackOvsMixin class and use it on ofagent

Change-Id: Ic7e3cd39345eed085aee7ae5f0a00368c5043258
use L2populationRpcCallBackOvsMixin on ovs agent

Change-Id: I4cbdab7f1297fb050fa2bcafa04f9b83bb15b3f8