JCLOUDS-781 cluster regions is a subset of regions.

mild cleanup, but major change is to make sure the cluster region list is a subset of regions.

Manually tested by invoking AWSEC2TemplateBuilderLiveTest with `-Djclouds.regions=us-east-1`. Verified in jclouds-wire.log that no calls were made to other regions. Mock test will happen in a future change.