### 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.
DEBUG_ANALYTICS = is_env_true("DEBUG_ANALYTICS")
SKIP_INFRA_DOWNLOADS = os.environ.get("SKIP_INFRA_DOWNLOADS", "").strip()
LOCAL_PORT_STEPFUNCTIONS = int(os.environ.get("LOCAL_PORT_STEPFUNCTIONS") or 8083)
STEPFUNCTIONS_LAMBDA_ENDPOINT = os.environ.get("STEPFUNCTIONS_LAMBDA_ENDPOINT", "").strip()
WINDOWS_DOCKER_MOUNT_PREFIX = os.environ.get("WINDOWS_DOCKER_MOUNT_PREFIX", "/host_mnt")
MAIN_CONTAINER_NAME = os.environ.get("MAIN_CONTAINER_NAME", "").strip() or "localstack_main"
LOCALSTACK_BUILD_GIT_HASH = os.environ.get("LOCALSTACK_BUILD_GIT_HASH", "").strip() or None
LOCALSTACK_BUILD_DATE = os.environ.get("LOCALSTACK_BUILD_DATE", "").strip() or None
S3_SKIP_SIGNATURE_VALIDATION = is_env_not_false("S3_SKIP_SIGNATURE_VALIDATION")
FORCE_SHUTDOWN = is_env_not_false("FORCE_SHUTDOWN")
OVERRIDE_IN_DOCKER = is_env_true("OVERRIDE_IN_DOCKER")
def has_docker():
    try:
        with open(os.devnull, "w") as devnull:
            subprocess.check_output("docker ps", stderr=devnull, shell=True)
        return True
    except Exception:
        return False
def is_linux():
    return platform.system() == "Linux"
LAMBDA_EXECUTOR = os.environ.get("LAMBDA_EXECUTOR", "").strip()
if not LAMBDA_EXECUTOR:
    LAMBDA_EXECUTOR = "docker"
    if not has_docker():
        LAMBDA_EXECUTOR = "local"
LAMBDA_FALLBACK_URL = os.environ.get("LAMBDA_FALLBACK_URL", "").strip()
LAMBDA_FORWARD_URL = os.environ.get("LAMBDA_FORWARD_URL", "").strip()
KINESIS_INITIALIZE_STREAMS = os.environ.get("KINESIS_INITIALIZE_STREAMS", "").strip()
CONFIG_ENV_VARS = [
    "SERVICES",