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
Remove enet from control-node Sconscript
Merge pull request #13 from Juniper/evpn-devel

Sync with evpn-devel.