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"
                },
...
```
MyFunc:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: %s
      Handler: index.handler
      Runtime:
        Ref: LambdaRuntime
      InlineCode: |
        def handler(event, context):
            return {'hello': 'world'}
"""
TEST_TEMPLATE_5 = """
AWSTemplateFormatVersion: 2010-09-09
Resources:
  S3Setup:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: test-%s
""" % short_uid()
TEST_ARTIFACTS_BUCKET = 'cf-artifacts'
TEST_ARTIFACTS_PATH = 'stack.yaml'
TEST_TEMPLATE_6 = """
AWSTemplateFormatVersion: 2010-09-09
Resources:
  NestedStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: http://localhost:4572/%s/%s
""" % (TEST_ARTIFACTS_BUCKET, TEST_ARTIFACTS_PATH)
def bucket_exists(name):