This field is optional. It controls what S3 API endpoint to connect to. By default https://s3.amazonaws.com/ is used (note that this works with all amazonaws zones - s3 transparently redirects between zones). When using Ubuntu Enterprise Cloud (Eucalyptus) the default URL should be something like http://cluster-controller-ip:8773/services/Walrus. For other EC2/S3 compatible servers please see your product documentation.