Get more properties of RDS instances

Introduce new key `rds_instances` that will return more properties provided by the AWS API in the form of a hash.

The following properties are returned:

* DB instance identifier
* DB endpoint (address + port)
* DB engine

Note on backward compatibility: The `rds` key is still supported and will continue to return the endpoint address only. We may remove it after migrating to `rds_instances`.