provider/aws: Fall back to old tagging mechanism for AWS gov and aws China (#14627)

Fixes: #14535

When in a `restricted` cloud, we should fall back to the old method of
tagging. Before this change we saw the following:

```
% terraform apply                                                                                                                                                                                         ✭
aws_instance.foo: Creating...
  ami:                          "" => "ami-0fa3c42c"
  associate_public_ip_address:  "" => "<computed>"
  availability_zone:            "" => "<computed>"
  ebs_block_device.#:           "" => "<computed>"
  ephemeral_block_device.#:     "" => "<computed>"
  instance_state:               "" => "<computed>"
  instance_type:                "" => "m1.small"
  ipv6_address_count:           "" => "<computed>"
  ipv6_addresses.#:             "" => "<computed>"
  key_name:                     "" => "<computed>"
  network_interface.#:          "" => "<computed>"
  network_interface_id:         "" => "<computed>"
  placement_group:              "" => "<computed>"
  primary_network_interface_id: "" => "<computed>"
  private_dns:                  "" => "<computed>"
  private_ip:                   "" => "<computed>"
  public_dns:                   "" => "<computed>"
  public_ip:                    "" => "<computed>"
  root_block_device.#:          "" => "<computed>"
  security_groups.#:            "" => "<computed>"
  source_dest_check:            "" => "true"
  subnet_id:                    "" => "<computed>"
  tags.%:                       "" => "1"
  tags.foo:                     "" => "bar"
  tenancy:                      "" => "<computed>"
  volume_tags.%:                "" => "<computed>"
  vpc_security_group_ids.#:     "" => "<computed>"
aws_instance.foo: Creation complete (ID: i-0009f227ae24791b9)

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

% terraform plan                                                                                                                                                                                          ✭
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

aws_instance.foo: Refreshing state... (ID: i-0009f227ae24791b9)
The Terraform execution plan has been generated and is shown below.
Resources are shown in alphabetical order for quick scanning. Green resources
will be created (or destroyed and then created if an existing resource
exists), yellow resources are being changed in-place, and red resources
will be destroyed. Cyan entries are data sources to be read.

Note: You didn't specify an "-out" parameter to save this plan, so when
"apply" is called, Terraform can't guarantee this is what will execute.

~ aws_instance.foo
    tags.%:   "0" => "1"
    tags.foo: "" => "bar"

Plan: 0 to add, 1 to change, 0 to destroy.
```

After this patch, we see the following:

```
% terraform apply                                                                                                                                                                                       ✹ ✭
[WARN] /Users/stacko/Code/go/bin/terraform-provider-aws overrides an internal plugin for aws-provider.
  If you did not expect to see this message you will need to remove the old plugin.
  See https://www.terraform.io/docs/internals/internal-plugins.html
aws_instance.foo: Creating...
  ami:                          "" => "ami-0fa3c42c"
  associate_public_ip_address:  "" => "<computed>"
  availability_zone:            "" => "<computed>"
  ebs_block_device.#:           "" => "<computed>"
  ephemeral_block_device.#:     "" => "<computed>"
  instance_state:               "" => "<computed>"
  instance_type:                "" => "m1.small"
  ipv6_address_count:           "" => "<computed>"
  ipv6_addresses.#:             "" => "<computed>"
  key_name:                     "" => "<computed>"
  network_interface.#:          "" => "<computed>"
  network_interface_id:         "" => "<computed>"
  placement_group:              "" => "<computed>"
  primary_network_interface_id: "" => "<computed>"
  private_dns:                  "" => "<computed>"
  private_ip:                   "" => "<computed>"
  public_dns:                   "" => "<computed>"
  public_ip:                    "" => "<computed>"
  root_block_device.#:          "" => "<computed>"
  security_groups.#:            "" => "<computed>"
  source_dest_check:            "" => "true"
  subnet_id:                    "" => "<computed>"
  tags.%:                       "" => "1"
  tags.foo:                     "" => "bar"
  tenancy:                      "" => "<computed>"
  volume_tags.%:                "" => "<computed>"
  vpc_security_group_ids.#:     "" => "<computed>"
aws_instance.foo: Creation complete (ID: i-04cd122e28f167a14)

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

% terraform plan                                                                                                                                                                                        ✹ ✭
[WARN] /Users/stacko/Code/go/bin/terraform-provider-aws overrides an internal plugin for aws-provider.
  If you did not expect to see this message you will need to remove the old plugin.
  See https://www.terraform.io/docs/internals/internal-plugins.html
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

aws_instance.foo: Refreshing state... (ID: i-04cd122e28f167a14)
No changes. Infrastructure is up-to-date.

This means that Terraform did not detect any differences between your
configuration and real physical resources that exist. As a result, Terraform
doesn't need to do anything.
```

builtin/providers/aws/resource_aws_instance.go

@@ -732,7 +732,7 @@ func resourceAwsInstanceUpdate(d *schema.ResourceData, meta interface{}) error {
 	restricted := meta.(*AWSClient).IsGovCloud() || meta.(*AWSClient).IsChinaCloud()
 
 	if d.HasChange("tags") {
-		if !d.IsNewResource() || !restricted {
+		if !d.IsNewResource() || restricted {
 			if err := setTags(conn, d); err != nil {
 				return err
 			} else {