### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

config.py's VALID_REGIONS only uses the default boto3 argument to find available regions. This means that regions from other partitions besides `aws` are not validated. 

When trying to list SNS Topics I get the error:

`An error occurred (InvalidParameter) when calling the ListTopics operation: Invalid region specified in "Authorization" header: "us-gov-west-1"`

I believe this is because VALID_REGIONS is set to use the default boto3 `get_available_regions()` parameter for `partition_name`, which is only 'aws'

https://github.com/localstack/localstack/blob/5784742b6f202133de11ef19544dd53c11c126e1/localstack/config.py#L454

https://github.com/boto/boto3/blob/master/boto3/session.py#L159-L162



### Expected Behavior

Non-default partition regions are also acknowledged as valid regions, and an error is not thrown.

VALID_REGIONS is set to aggregate regions from all available partitions.

### How are you starting LocalStack?

Custom (please describe below)

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    Running through a kubernetes deployment, using the image localstack: 0.12.17

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

```
# using boto3 locally
sns_client: boto3.client = boto3.client("sns", endpoint_url=AWS_URL)
topics = sns_client.list_topics()
```

`An error occurred (InvalidParameter) when calling the ListTopics operation: Invalid region specified in "Authorization" header: "us-gov-west-1"`

### Environment

```markdown
- LocalStack Docker image: 0.12.17
- Kubernetes
- Running on govcloud
```


### Anything else?

Current workaround:

setting `USE_SINGLE_REGION=true` and `AWS_DEFAULT_REGION=us-east-1` successfully mitigates this problem, and causes no error to the thrown when listing topics.
os.environ.get("DYNAMODB_WRITE_ERROR_PROBABILITY", "").strip() or 0.0
)
DYNAMODB_HEAP_SIZE = os.environ.get("DYNAMODB_HEAP_SIZE", "").strip() or "256m"
HOSTNAME_EXTERNAL = os.environ.get("HOSTNAME_EXTERNAL", "").strip() or LOCALHOST
SQS_PORT_EXTERNAL = int(os.environ.get("SQS_PORT_EXTERNAL") or 0)
LOCALSTACK_HOSTNAME = os.environ.get("LOCALSTACK_HOSTNAME", "").strip() or LOCALHOST
HOSTNAME_FROM_LAMBDA = os.environ.get("HOSTNAME_FROM_LAMBDA", "").strip()
LAMBDA_REMOTE_DOCKER = is_env_true("LAMBDA_REMOTE_DOCKER")
BUCKET_MARKER_LOCAL = (
    os.environ.get("BUCKET_MARKER_LOCAL", "").strip() or DEFAULT_BUCKET_MARKER_LOCAL
)
LAMBDA_DOCKER_NETWORK = os.environ.get("LAMBDA_DOCKER_NETWORK", "").strip()
LAMBDA_DOCKER_DNS = os.environ.get("LAMBDA_DOCKER_DNS", "").strip()
LAMBDA_DOCKER_FLAGS = os.environ.get("LAMBDA_DOCKER_FLAGS", "").strip()
LAMBDA_CONTAINER_REGISTRY = (
    os.environ.get("LAMBDA_CONTAINER_REGISTRY", "").strip() or DEFAULT_LAMBDA_CONTAINER_REGISTRY
)
LAMBDA_REMOVE_CONTAINERS = (
    os.environ.get("LAMBDA_REMOVE_CONTAINERS", "").lower().strip() not in FALSE_STRINGS
)
DATA_DIR = os.environ.get("DATA_DIR", "").strip()
TMP_FOLDER = os.path.join(tempfile.gettempdir(), "localstack")
for folder in [DATA_DIR, TMP_FOLDER]:
    if folder and not os.path.exists(folder):
        try:
            os.makedirs(folder)
        except Exception:
            
            
            pass