Feature 10887: Add cross-model relations support

Changes:
- Update the `osm_lcm.ns.NsLcm._add_vca_relations` function, and
decouple it into simpler and smaller functions.
- Get relation data for the descriptor taking into account the changes
introduced here: https://osm.etsi.org/gerrit/11212/


- Add `osm_lcm.data_utils.vca` module to take care of VCA related parts
in the IM
- Add a couple of functions into the `osm_lcm.data_utils` package:
  - nsd: get_vnf_profile(), get_ns_configuration(),
get_ns_configuration_relation_list()
  - nsr: get_nsd(), get_deployed_vca_list(), get_deployed_vca()
  - vnfd: get_relation_list()
- Rename `osm_lcm.data_utils.vnfd.get_kdu_profile` to `osm_lcm.data_utils.vnfd.get_kdu_resource_profile`

Change-Id: I6da29e656d092e17c85b44f5b3960a6ca3aa3ad8
Signed-off-by: David Garcia <david.garcia@canonical.com>