Support of VNF and NS relations as described in pad 6297

This adds the required changes in the IM to replace
the previous structure of relations by a new one following the
design for feature6297 in:
https://osm.etsi.org/pad/p/feature6297



Change-Id: I57f491a74d5e6e87158554421735512d0c5ec4e6
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>