Allow users to specify an ebs type

/cc @opscode/release-engineers 
Should this default to standard? The [ebs volume documentation](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSVolumeTypes.html) doesn't appear to indicate the default, but "standard" does sound like a default. From [the pricing](http://aws.amazon.com/ebs/pricing/) standard is the cheaper option as well.