Update vnfc-relations

Renamed `vnfc-relations` to `relations`

Change entities content:
- Replace ambiguous `id` with `vnf-profile-id`, `kdu-resource-profile-id` and
`vdu-profile-id`
- Add `execution-environment-ref` to select the ee from the potential
list
- Change `entities` from a two-element list to a `provider` and
`requirer` containers. This makes the IM more robust and avoids
unexpected list of endpoints with more than two elements.

Related feature: 10887 (features#10887

)

Change-Id: I327764adc5366427d6366e1609f7aebad878c4a6
Signed-off-by: David Garcia <david.garcia@canonical.com>