Use default availability zone when creating EBS volumes

Presently, in the `EC2Connection.create_volume` method, `zone` is a required parameter despite the fact that the [CreateVolume](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateVolume.html) describes it as being optional. Thus, with this patch, we only include it if it's not `None`.