Normalize route origin when learning routes from a VM/VNF

This feature request is related to BGP as a Service (i.e. the vRouter peering with a VNF running in a VM) and concerns the route origin field in BGP. On PNF routers it is possible to override the route origin field of incoming routes that are learned from peers. This same capability is needed when Contrail learns routes from VNFs.

In particular this is related to the vLB VNF because VNF does NOT want route origin to be part of the tiebreaker protocol when choosing between routes. It is necessary to be able to set the route origin field of routes learned from vLB VNFs to a single value across AT&T's entire network so that differences in route origin value between different vLB instances won't contribute to route selection. Needed for VNF’s using Anycast IP’s.