Update acceptance test to use t2.micro instances.

The AWS free tier allows up to 750 hours on t2.micro
instance types. It's better to use cheaper instances
in case the resources are not cleaned up if a tests
is canceled or crashes.

builtin/providers/aws/resource_aws_autoscaling_group_test.go

@@ -627,7 +627,7 @@ resource "aws_autoscaling_group" "bar" {
 const testAccAWSAutoScalingGroupConfig_withPlacementGroup = `
 resource "aws_launch_configuration" "foobar" {
   image_id = "ami-21f78e11"
-  instance_type = "c3.large"
+  instance_type = "t2.micro"
 }
 
 resource "aws_placement_group" "test" {