Agent EVPN multicast support

1) Add EVPN composite NH in L2 composite NH.
2) Populate EVPN NH with olist sent by control node.
3) Add changes to support OlistType(xmpp_enet.xsd) for receiving EVPN NH.
4) Create Interface Composite NH for L2 and L3 multicast composites. (L2
   composite = Fabric CNH, EVPN CNH, Interface CNH) and L3 composite = Fabric CNH,
   Interface CNH)
5) Add MPLS label creation for EVPN multicast.
6) Change subnet route nexthop to EVPN composite NH from current discard.
7) Test cases