Add ebs_optimized attribute

Make it possible to spawn test instances with EBS optimized storage.

    driver_config:
      ...
      flavor_id: m1.large
      ebs_optimized: true

Will throw following exception if you try to spawn unsupported instances with EBS optimization:

    Class: Kitchen::ActionFailed
    Message: Unsupported => Your requested instance type (m1.small) is not supported in this region (eu-west-1). Please retry your request in one of the following regions: us-east-1.