1. A method of operating a Network Function Virtualization (NFV) Management and Orchestration (MANO) data communication system to support an Application Layer Traffic Offload (ALTO) system for a Network Service (NS), the method comprising: MANO computer system circuitry processing an NFV MANO orchestrator and an NFV MANO Virtualized Infrastructure Manager (VIM) and responsively exchanging network data to support the NS; the MANO computer system circuitry processing the NFV MANO orchestrator and an NFV MANO Virtual Network Function Manager (VNFM) and responsively exchanging function data to support the NS; the MANO computer system circuitry processing the NFV MANO VIM and responsively exchanging the network data with NFV Infrastructures (NFVIs) that execute Virtual Machines (VMs) that instantiate Virtual Network Function (VNFs) to support the NS; the MANO computer system circuitry processing the NFV MANO VNFM and responsively exchanging function data with the VNFs to support the NS; the MANO computer system circuitry processing the NFV MANO orchestrator and responsively indexing some of the network data and the function data to generate NFV ALTO topology data that associates individual ALTO Provider Identifiers (PIDs) with individual ones of the NFVIs, that associates the individual ALTO PIDs with individual ones of the NFV VNFs, that associates an ALTO external domain with individual NFV Virtual Links (VLs), that associates the individual ones of the NFV VLs with individual NFVI resource usage, and that associates the individual ones of the NFV VLs with individual VL usage costs; and the MANO computer system circuitry processing the NFV MANO orchestrator and responsively transferring the NFV ALTO topology data for delivery to the ALTO system.