Change models.json to configure existing models

Breaking change.

In the new 2.x project layout, definition of loopback Models is out of
scope of the boot process. The bootstrapper only configures existing
models - attaches them to a dataSource and the app object.
Remove auto-attach.

Breaking change.

The bootstrapper no longer calls `loopback.autoAttach`. Applications
have to explicitly configure datasources for their models
via `models.json`.