Ability to specify new SSD EBS option

AWS [rolled out general purpose SSD volumes](http://aws.amazon.com/blogs/aws/new-ssd-backed-elastic-block-storage/) in June 2014 as the default storage volume type for EBS. This PR keeps the previous default ("standard" magnetic drives) but it also exposes volume_type as an option, allowing the selection of the SSD (i.e. "gp2") volume type.