Rework the mechanism for creating twins (and virtual realnet)

- rtdt-manager.go:
    - LaunchRealnetVEnv() to take over functionality previously in
      main.go and derive realnet clab configuration via
      ProduceClabConfig()
    - ProduceClabConfig() merge a base clab config (gosdn, rabbitmq, mongo
      and so on) with a passed-in sdnconfig json to create the environment
- clab-config.go:
    - Make LoadConfig take full path like WriteConfig
    - Implement mechanism to get templates of nodes that give
      information not retrievable from the running containerlab
      environment (nodeBlueprints and GetNodeTemplate())
- venv.go:
    - CreateDevices() now uses sdnconfig networkElements to create
      devices