Preserve property order in models.json

388847 added `public` as a property to `ModelConfig`, which had an
unintended side-effect of changing the order of properties in
`model-config.json`. Before that change, `dataSource` was written
before `public`. After that change, `public` is first.

This commit fixes the problem by explicitly defining `dataSource`
property in ModelConfig definition. (The property is otherwise added
by "belongsTo DataSource" relation.)