[CT-2195] [Feature] Validation: disallow access:private on ungrouped model

[MichelleArk]

Models without a group, and with access set to private should raise an error at parse-time.

models: 
- name: model
  access: private

ParsedNode.patch could be the right spot for this validation, since group information should be available at this point.

[gshank]

When ParsedNode.patch is called the group might not be set yet. "patch" is called before "update_node_config", so access is set before group. This is another one of those problems of having to coordinate validation across nodes and node configs. Which actually might be simplified if we get rid of separate node configs.

[jtcohen6]

I don't think this is a MUST for Phase 1. It will save users some confusion if they try to ref an ungrouped private model, but it should yield the same result at the end of the day, which is that private models are useless unless in an explicit group.

[MichelleArk]

Closing as not planned with the same rationale as #7687:

@jtcohen6: I don't expect people to run into this, unless they're trying to do interesting things, and we shouldn't stand in their way within the code. We should document this as a limitation, and we can revisit if it causes major confusion / UX issues.