### 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.
global SERVICE_PORTS, CONFIG_ENV_VARS
    SERVICE_PORTS = service_ports or parse_service_ports()
    globs = globals()
    protocol = get_protocol()
    
    for key, value in six.iteritems(DEFAULT_SERVICE_PORTS):
        key_upper = key.upper().replace("-", "_")
        
        port_var_name = "PORT_%s" % key_upper
        port_number = service_port(key)
        globs[port_var_name] = port_number
        url = "%s://%s:%s" % (protocol, LOCALSTACK_HOSTNAME, port_number)
        
        url_key = "TEST_%s_URL" % key_upper
        
        existing = os.environ.get(url_key)
        url = existing or url
        
        globs[url_key] = url
        
        os.environ[url_key] = url
    
    os.environ["LOCALSTACK_HOSTNAME"] = LOCALSTACK_HOSTNAME
    
    CONFIG_ENV_VARS += [
        "LOCALSTACK_" + v for v in CONFIG_ENV_VARS if not v.startswith("LOCALSTACK_")
    ]
    CONFIG_ENV_VARS = list(set(CONFIG_ENV_VARS))
def service_port(service_key):