EKS-A should substitute relative vSphere network names with absolute path

Since networks are immutable, datacenter fields in EKS-A should figure out full absolute path and substitute them. This should be done during pre-flight validation.

Locate the pre-flight validation entry point for EKS-A datacenter fields and trace how vSphere network names are handled. The work is done when relative network names are resolved to absolute paths during validation before the immutable networks are used; the issue names no specific files or tests, so identify the relevant validation coverage first.

Written by the indexing model from the issue text.