Add relations between the VNFs and VDUs

[oEscal]

VNFs at the NS level, and VDUs at the VNF level. This will allow to launch the charmed OSM on the same VNF as the one where the corresponding K8s cluster is launched (this will end up as being an HNF). This will also allow to just add a new cluster (the other type of VNF) just when the charmed OSM (of the other VNF) is ready