This field is optional. It controls what EC2 API endpoint to connect to. By default https://us-east-1.ec2.amazonaws.com/ is used. To use other Amazon AWS zones, change this appropriately. When using Ubuntu Enterprise Cloud (Eucalyptus) a url like http://cluster-controler:8773/services/Eucalyptus is needed. For other EC2 API-compatible clouds please see your product documentation.