Support for Cloud Databases datastores and configurations

This PR introduces support for:
* [Datastores](http://docs.rackspace.com/cdb/api/v1.0/cdb-devguide/content/datastore_types_and_versions.html),
* [Configurations](http://docs.rackspace.com/cdb/api/v1.0/cdb-devguide/content/configurations.html), and
* Updating a database instance to use a configuration.