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

PR for TheWeatherCompany/grid-api#1212
Hi @clstokes I've added it in separate class as it's different store than EBS volumes and as I understood there is no API to get info about _ephemeral_ direves, it can be checked only by going directly to the instance and retrieve its _metadata_. Anyway I'll replace it...