Only validate dataSource when defined (Fixes #482)
registry: warn when dataSource is not specified

Modify `registry.configureModel()` to log a warning when `dataSource`
optiont is not specified at all.

Users should provide `dataSource: null` when the model is intentionally
not attached to any data-source.
Merge pull request #483 from strongloop/fix/undefined-datasource

Only validate dataSource when defined