Add support for VolumeType parameter to run_instances()

We can now have SSD root devices, by specifying "gp2" as for the BlockDeviceMapping.n.Ebs.VolumeType parameter to run_instances():

http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-RunInstances.html

This commit adds the ability to send that parameter via Net::Amazon::EC2