Fixed api response of attach_volume and detach_volume methods according to aws api docs.

Fixed api response of ec2 attach_volume and detach_volume methods according to aws api docs.
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-AttachVolume.html
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DetachVolume.html