TheWeatherCompany/grid-api#1212 Add support for instance-store/ephemeral drives.

PR for TheWeatherCompany/grid-api#1212
I think we should add *ephemeral* as an option [here](https://github.com/TheWeatherCompany/dasein-cloud-aws/blob/ccd16662e7ac31dec46c81e58865525c24a172c9/src/main/java/org/dasein/cloud/aws/compute/EBSVolume.java#L278-L280) rather than have a separate constants class for it.