Make availability zones a computed attribute.

Fixes a diff calculation error when only a VPC zone
identifiers is provided. In this case the associated
availability zones are computed from the subnets per
the AWS documentation.

builtin/providers/aws/resource_aws_autoscaling_group.go

@@ -92,6 +92,7 @@ func resourceAwsAutoscalingGroup() *schema.Resource {
 			"availability_zones": &schema.Schema{
 				Type:     schema.TypeSet,
 				Optional: true,
+				Computed: true,
 				Elem:     &schema.Schema{Type: schema.TypeString},
 				Set:      schema.HashString,
 			},