OVSDB Enhancements

1. Modify the logic used to identify routes that must be exported to controller
   In existing design, controller-peer looks at the peer-type to decide if
   path added by it can be exported to controller. Any new type of peer will
   need modifying this code as well.

   The design is modified such that, a virtual function in peer specifies if
   the routes added by peer should be exported to controller.

2. Modify the logic to set the nexthop-ip in route exported to controller.
   In existing design, agent always sets IP address of VHOST to be the
   nexthop-ip in the route exported to controller.

   In case of OVS, agent is exporting route on-behalf of the ToR. In this
   case the nexthop should actually be that of ToR. The design is modified
   to get the nexthop-ip from peer

   Only EVPN routes are using the new API. L3 and Mcast routes will be
   modified later to use the new API.

3. Add skeletal code for UT of OVS-Peer