Add heartbeat_timeout validation function in autoscaling lifecycle hook

Noticed that when trying to set a value lower than 30 seconds and got rejected at during `terraform apply`. Adding that to nicely fail during `terraform plan` when that happens.

This is not documented in [ec2/APIReference/API_LifecycleHook](https://docs.aws.amazon.com/autoscaling/ec2/APIReference/API_LifecycleHook.html) but can be found in the [Autoscaling user guide](https://github.com/awsdocs/amazon-ec2-auto-scaling-user-guide/blob/master/doc_source/lifecycle-hooks.md#add-lifecycle-hooks-console)

aws/resource_aws_autoscaling_lifecycle_hook.go

@@ -49,8 +49,9 @@ func resourceAwsAutoscalingLifecycleHook() *schema.Resource {
 				ValidateFunc: validation.StringInSlice([]string{"CONTINUE", "ABANDON"}, false),
 			},
 			"heartbeat_timeout": {
-				Type:     schema.TypeInt,
-				Optional: true,
+				Type:         schema.TypeInt,
+				Optional:     true,
+				ValidateFunc: validation.IntBetween(30, 7200),
 			},
 			"lifecycle_transition": {
 				Type:         schema.TypeString,