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"
                },
...
```
import os
import json
import unittest
from localstack.constants import TEST_AWS_ACCOUNT_ID
from localstack.utils.aws import aws_stack
from localstack.utils.common import load_file, retry, short_uid, to_str
from localstack.utils.cloudformation import template_deployer
from botocore.exceptions import ClientError
from botocore.parsers import ResponseParserError
THIS_FOLDER = os.path.dirname(os.path.realpath(__file__))
TEST_TEMPLATE_1 = os.path.join(THIS_FOLDER, 'templates', 'template1.yaml')
TEST_TEMPLATE_2 = os.path.join(THIS_FOLDER, 'templates', 'template2.yaml')
TEST_TEMPLATE_3 = """
AWSTemplateFormatVersion: 2010-09-09
Resources:
  S3Setup:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: test-%s
""" % short_uid()
TEST_TEMPLATE_4 = """
AWSTemplateFormatVersion: 2010-09-09
Transform: AWS::Serverless-2016-10-31
Parameters:
  LambdaRuntime:
    Type: String
    Default: python3.6
Resources: