Fix initialization of instances array in AWS autoscaling group

I'm not sure why I left the instances array in the old style in #2161, but @josegonzalez pointed out that it was causing the instances collection to be empty.

Wasn't sure how to really test this, since the mock autoscaling group doesn't actually create instances, and implementing that seemed like a larger project than I'm ready to take on right now!  I did run it against a live autoscaling group with instances though, and confirmed that they are showing up in the instance collection.