Given a CF template that defines outputs as named exports, I expect the "describe-stack" endpoint to include the exported names. With localstack, that is currently not the case.

Example template:
```yaml
  Outputs:
    VPCId:
      Value: {Ref: MyVPC}
      Export:
        Name: My-VPCId

  Resources:
    MyVPC:
      Properties:
        CidrBlock: 10.0.0.0/16
      Type: AWS::EC2::VPC

```

AWS:
```
$ aws cloudformation describe-stacks
...
                {
                    "OutputKey": "VPCId",
                    "OutputValue": "vpc-0123456789",
                    "ExportName": "My-VPCId"
                },

```

localstack:
```
$ aws cloudformation --endpoint-url=http://localhost:4581/ describe-stacks
...
                {
                    "OutputKey": "VPCId",
                    "OutputValue": "vpc-0123456789"
                },
...
```
try:
            if attribute_name == 'StreamArn':
                streams = aws_stack.connect_to_service('dynamodbstreams').list_streams(TableName=self.name)['Streams']
                return streams[0]['StreamArn'] if streams else None
            return DynamoDB_Table_get_cfn_attribute_orig(self, attribute_name)
        except Exception as e:
            LOG.warning('Unable to get attribute "%s" from resource %s: %s' % (attribute_name, type(self), e))
            raise
    DynamoDB_Table_get_cfn_attribute_orig = dynamodb_models.Table.get_cfn_attribute
    dynamodb_models.Table.get_cfn_attribute = DynamoDB_Table_get_cfn_attribute
    
    def IAM_Role_get_cfn_attribute(self, attribute_name):
        try:
            return IAM_Role_get_cfn_attribute_orig(self, attribute_name)
        except Exception:
            if attribute_name == 'Arn':
                return aws_stack.role_arn(self.name)
            raise
    IAM_Role_get_cfn_attribute_orig = iam_models.Role.get_cfn_attribute
    iam_models.Role.get_cfn_attribute = IAM_Role_get_cfn_attribute
    
    def SNS_Topic_get_cfn_attribute(self, attribute_name):