### 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.
break
try:
    DOCKER_HOST_FROM_CONTAINER = DOCKER_BRIDGE_IP
    if not is_in_docker and not is_in_linux:
        
        
        if LOCALSTACK_HOSTNAME == LOCALHOST:
            DOCKER_HOST_FROM_CONTAINER = "host.docker.internal"
    
    if is_in_docker:
        DOCKER_HOST_FROM_CONTAINER = socket.gethostbyname("host.docker.internal")
        if LOCALSTACK_HOSTNAME == DOCKER_BRIDGE_IP:
            LOCALSTACK_HOSTNAME = DOCKER_HOST_FROM_CONTAINER
except socket.error:
    pass
if is_in_docker and not os.environ.get("LAMBDA_REMOTE_DOCKER", "").strip():
    LAMBDA_REMOTE_DOCKER = True
CONFIG_FILE_PATH = os.path.join(TMP_FOLDER, ".localstack")
if not is_in_docker:
    CONFIG_FILE_PATH = os.path.join(expanduser("~"), ".localstack")
no_proxy = ",".join(set((LOCALSTACK_HOSTNAME, LOCALHOST, LOCALHOST_IP, "[::1]")))
if os.environ.get("no_proxy"):
    os.environ["no_proxy"] += "," + no_proxy
elif os.environ.get("NO_PROXY"):
    os.environ["NO_PROXY"] += "," + no_proxy
else:
    os.environ["no_proxy"] = no_proxy
CLI_COMMANDS = {}
VALID_REGIONS = set(Session().get_available_regions("sns"))
def parse_service_ports():