fix return code for deleteIfEmpty

starts fixing tests in https://issues.apache.org/jira/browse/JCLOUDS-641
From the [API docs](http://docs.openstack.org/api/openstack-object-storage/1.0/content/DELETE_deleteContainer__v1__account___container__storage_container_services.html), it seems that 409 is returned when the container is **not** empty.

This change now returns `false` in that case, if I understand it correctly. In that case, the Javadoc should be changed?