EC2 Network Interface: Copy over device index

This request applies #1632 by @bobveznat with the following changes:

 1. Cast `device_index` to `int`
 1. Add a unit test to make sure the value is copied over properly

According to http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeNetworkInterfaces.html the field should be an integer, so I think we should treat it as such.