provider/aws: fix force_delete on ASGs

The `ForceDelete` parameter was getting sent to the upstream API call,
but only after we had already finished draining instances from
Terraform, so it was a moot point by then.

This fixes that by skipping the drain when force_delete is true, and it
also simplifies the field config a bit:

 * set a default of false to simplify the logic
 * remove `ForceNew` since there's no need to replace the resource to
   flip this value
 * pull a detail comment from code into the docs

builtin/providers/aws/resource_aws_autoscaling_group.go

@@ -73,8 +73,7 @@ func resourceAwsAutoscalingGroup() *schema.Resource {
 			"force_delete": &schema.Schema{
 				Type:     schema.TypeBool,
 				Optional: true,
-				Computed: true,
-				ForceNew: true,
+				Default:  false,
 			},
 
 			"health_check_grace_period": &schema.Schema{
@@ -334,15 +333,9 @@ func resourceAwsAutoscalingGroupDelete(d *schema.ResourceData, meta interface{})
 	}
 
 	log.Printf("[DEBUG] AutoScaling Group destroy: %v", d.Id())
-	deleteopts := autoscaling.DeleteAutoScalingGroupInput{AutoScalingGroupName: aws.String(d.Id())}
-
-	// You can force an autoscaling group to delete
-	// even if it's in the process of scaling a resource.
-	// Normally, you would set the min-size and max-size to 0,0
-	// and then delete the group. This bypasses that and leaves
-	// resources potentially dangling.
-	if d.Get("force_delete").(bool) {
-		deleteopts.ForceDelete = aws.Bool(true)
+	deleteopts := autoscaling.DeleteAutoScalingGroupInput{
+		AutoScalingGroupName: aws.String(d.Id()),
+		ForceDelete:          aws.Bool(d.Get("force_delete").(bool)),
 	}
 
 	// We retry the delete operation to handle InUse/InProgress errors coming
@@ -414,6 +407,11 @@ func getAwsAutoscalingGroup(
 func resourceAwsAutoscalingGroupDrain(d *schema.ResourceData, meta interface{}) error {
 	conn := meta.(*AWSClient).autoscalingconn
 
+	if d.Get("force_delete").(bool) {
+		log.Printf("[DEBUG] Skipping ASG drain, force_delete was set.")
+		return nil
+	}
+
 	// First, set the capacity to zero so the group will drain
 	log.Printf("[DEBUG] Reducing autoscaling group capacity to zero")
 	opts := autoscaling.UpdateAutoScalingGroupInput{

website/source/docs/providers/aws/r/autoscaling_group.html.markdown

@@ -57,7 +57,10 @@ The following arguments are supported:
     for this number of healthy instances all attached load balancers.
     (See also [Waiting for Capacity](#waiting-for-capacity) below.)
 * `force_delete` - (Optional) Allows deleting the autoscaling group without waiting
-   for all instances in the pool to terminate.
+   for all instances in the pool to terminate.  You can force an autoscaling group to delete
+   even if it's in the process of scaling a resource. Normally, Terraform
+   drains all the instances before deleting the group.  This bypasses that
+   behavior and potentially leaves resources dangling.
 * `load_balancers` (Optional) A list of load balancer names to add to the autoscaling
    group names.
 * `vpc_zone_identifier` (Optional) A list of subnet IDs to launch resources in.