1382751 prevent non subordinate container scoped relations

This fixes LP #1382751.

Container scoped relations are now only allowed if there's exactly one subordinate service involved. This prevents conditions that should never have been allowed. The Uniter errors and exits when it encounters relations without a subordinate service on one side.

InferEndpoints now ignores container relations unless there's exactly one subordinate service involved.
This line should probably also be greater than one