Update ElasticHosts pre-installed images and added new regions

https://issues.apache.org/jira/browse/JCLOUDS-517

* Added new ElasticHosts regions: Amsterdam, San José, Hong Kong and Sydney
* Fixed the pre-installed image list according to: http://www.elastichosts.com/support/api/ (see the "Standard drives" section)
* Fixed a minor issue in the ComputeService: before starting to image a drive, we have to wait until it is not claimed.