provider/aws: Allow aws_instances to be resized rather than forcing a

new instance

Fixes: #9157

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSInstance_changeInstanceType'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/16 15:13:21 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_changeInstanceType -timeout 120m
=== RUN   TestAccAWSInstance_changeInstanceType
--- PASS: TestAccAWSInstance_changeInstanceType (303.85s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	303.876s
```

builtin/providers/aws/resource_aws_instance.go

@@ -63,7 +63,6 @@ func resourceAwsInstance() *schema.Resource {
 			"instance_type": {
 				Type:     schema.TypeString,
 				Required: true,
-				ForceNew: true,
 			},
 
 			"key_name": {
@@ -410,7 +409,7 @@ func resourceAwsInstanceCreate(d *schema.ResourceData, meta interface{}) error {
 		*instance.InstanceId)
 
 	stateConf := &resource.StateChangeConf{
-		Pending:    []string{"pending"},
+		Pending:    []string{"pending", "stopped"},
 		Target:     []string{"running"},
 		Refresh:    InstanceStateRefreshFunc(conn, *instance.InstanceId),
 		Timeout:    10 * time.Minute,
@@ -606,6 +605,60 @@ func resourceAwsInstanceUpdate(d *schema.ResourceData, meta interface{}) error {
 		}
 	}
 
+	if d.HasChange("instance_type") {
+		log.Printf("[INFO] Stopping Instance %q for instance_type change", d.Id())
+		_, err := conn.StopInstances(&ec2.StopInstancesInput{
+			InstanceIds: []*string{aws.String(d.Id())},
+		})
+
+		stateConf := &resource.StateChangeConf{
+			Pending:    []string{"pending", "running", "shutting-down", "stopped", "stopping"},
+			Target:     []string{"stopped"},
+			Refresh:    InstanceStateRefreshFunc(conn, d.Id()),
+			Timeout:    10 * time.Minute,
+			Delay:      10 * time.Second,
+			MinTimeout: 3 * time.Second,
+		}
+
+		_, err = stateConf.WaitForState()
+		if err != nil {
+			return fmt.Errorf(
+				"Error waiting for instance (%s) to stop: %s", d.Id(), err)
+		}
+
+		log.Printf("[INFO] Modifying instance type %s", d.Id())
+		_, err = conn.ModifyInstanceAttribute(&ec2.ModifyInstanceAttributeInput{
+			InstanceId: aws.String(d.Id()),
+			InstanceType: &ec2.AttributeValue{
+				Value: aws.String(d.Get("instance_type").(string)),
+			},
+		})
+		if err != nil {
+			return err
+		}
+
+		log.Printf("[INFO] Starting Instance %q after instance_type change", d.Id())
+		_, err = conn.StartInstances(&ec2.StartInstancesInput{
+			InstanceIds: []*string{aws.String(d.Id())},
+		})
+
+		stateConf = &resource.StateChangeConf{
+			Pending:    []string{"pending"},
+			Target:     []string{"running"},
+			Refresh:    InstanceStateRefreshFunc(conn, d.Id()),
+			Timeout:    10 * time.Minute,
+			Delay:      10 * time.Second,
+			MinTimeout: 3 * time.Second,
+		}
+
+		_, err = stateConf.WaitForState()
+		if err != nil {
+			return fmt.Errorf(
+				"Error waiting for instance (%s) to become ready: %s",
+				d.Id(), err)
+		}
+	}
+
 	if d.HasChange("disable_api_termination") {
 		_, err := conn.ModifyInstanceAttribute(&ec2.ModifyInstanceAttributeInput{
 			InstanceId: aws.String(d.Id()),