OFAgent: Share codes of l2-population in OVS agent

This is step 1 implementation of OFAgent l2-population.
OFAgent partially uses codes in OVS agent on implementation of l2-population.
We share these codes adding mixin class for OVS to l2-population rpc.
We use a ryu library instead of executing a ovs-vsctl command
and on OFAgent l2-population is no longer optional.
Also a function of local arp responder will be implemented on step 2.

related commit: b6133c35dd587f6b01e8ec12757347b2767713a6

Partially implements: blueprint ofagent-l2pop

Change-Id: I99a2adfd380a9fefe34c53e0dabc21d8cf5936cc
Merge "OFAgent: Share codes of l2-population in OVS agent"