Use discoverSchema for the case where options.schema is not present

/to @ritch 

When options.schema is not passed, the discovery defaults to a given schema. The key of the returned definitions will be schema.table. Using options.schema +'.'+ table will not work.
`discoverSchema()` is a shortcut to `discoverSchemas()` that only returns one model definition. It's only possible for `discoverSchemas()` to return more than one models if we allow it to follow relations. `discoverSchema` always set `relations` flag to false.

See https://github.com/strongloop/loopback-datasource-juggler/blob/master/lib/datasource.js#L1089