### 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.
return EDGE_PORT_HTTP or EDGE_PORT
def get_edge_url(localstack_hostname=None, protocol=None):
    port = get_edge_port_http()
    protocol = protocol or get_protocol()
    localstack_hostname = localstack_hostname or LOCALSTACK_HOSTNAME
    return "%s://%s:%s" % (protocol, localstack_hostname, port)
populate_configs()
if DEBUG:
    logging.getLogger("").setLevel(logging.DEBUG)
    logging.getLogger("localstack").setLevel(logging.DEBUG)
BUNDLE_API_PROCESSES = True
def load_config_file(config_file=None):
    from localstack.utils.common import get_or_create_file, to_str
    config_file = config_file or CONFIG_FILE_PATH
    content = get_or_create_file(config_file)
    try:
        configs = json.loads(to_str(content) or "{}")
    except Exception as e:
        print("Unable to load local config file %s as JSON: %s" % (config_file, e))
        return {}
    return configs
if LS_LOG == "trace":
    load_end_time = time.time()
    LOG = logging.getLogger(__name__)
    LOG.debug(
        "Initializing the configuration took %s ms" % int((load_end_time - load_start_time) * 1000)
    )