Get instances by autoscaling group name instead of polling all instances

Was wondering if there was a reason its doing describe all instances instead of polling by Auto Scaling Group name using [describeAutoScalingGroups](http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/AutoScaling.html#describeAutoScalingGroups-property)?

Locate the code that describes or polls all instances, then read the AWS SDK describeAutoScalingGroups documentation linked in the issue. Update the instance lookup to use the autoscaling group name, and verify that instances are obtained without polling every instance.

Written by the indexing model from the issue text.