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"
                },
...
```
cloudformation = aws_stack.connect_to_service('cloudformation')
        
        stack_name = 'stack-%s' % short_uid()
        cloudformation.create_stack(StackName=stack_name, TemplateBody=TEST_TEMPLATE_3)
        
        response = cloudformation.create_change_set(StackName=stack_name,
            TemplateBody=TEST_TEMPLATE_3, ChangeSetName='nochanges')
        self.assertIn(':%s:changeSet/nochanges/' % TEST_AWS_ACCOUNT_ID, response['Id'])
        self.assertIn(':%s:stack/' % TEST_AWS_ACCOUNT_ID, response['StackId'])
    def test_sam_template(self):
        cloudformation = aws_stack.connect_to_service('cloudformation')
        awslambda = aws_stack.connect_to_service('lambda')
        
        stack_name = 'stack-%s' % short_uid()
        func_name = 'test-%s' % short_uid()
        template = TEST_TEMPLATE_4 % func_name
        cloudformation.create_stack(StackName=stack_name, TemplateBody=template)
        
        result = awslambda.invoke(FunctionName=func_name)
        result = json.loads(to_str(result['Payload'].read()))
        self.assertEqual(result, {'hello': 'world'})
    def test_nested_stack(self):