Try to implement topology, so that clabconfig is initially only used to

parse the passed-in configuration file and then upload to the DB. Twin
should then be derived from DB topology data. So far, implemented
everything up to UploadTopology(). Also split up NewVEnv() function into
multiple functions.
- venv.go: CreateDevices(), replaces code in NewVEnv(), uses rtdt_topology
instead of clabconfig.topology, add DeriveTopologyFromClabData(), add
UploadTopology()
- rtdt-topology.go: Add GettNodeByName() function, change Topology members
to references, implement converter functions to return api structs
- clab-config.go: Add GetNodeByName() function
- main.go: Use DeriveTopologyFromClabData() and UploadTopology()