Variable validation: `subscription_infra_map`

Add validation logic to all variables. Use validation to ensure that referential integrity is maintained ("foreign key" references to other resources (e.g., `location_name`) map back to "primary key" map variables (e.g., `locations`)). Ensure that mid-level `subscription_infra_map` variable validation logic aligns with lower-level `infra_map_vm_set` module.