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"
                },
...
```
from boto.cloudformation.stack import Output
from moto.cloudformation.models import FakeStack
from moto.cloudformation.exceptions import ValidationError, UnformattedGetAttTemplateException
from localstack import config
from localstack.constants import DEFAULT_PORT_CLOUDFORMATION_BACKEND, TEST_AWS_ACCOUNT_ID, MOTO_ACCOUNT_ID
from localstack.utils.aws import aws_stack, aws_responses
from localstack.utils.common import FuncThread, short_uid, recurse_object, clone, json_safe
from localstack.stepfunctions import models as sfn_models
from localstack.services.infra import (
    get_service_protocol, start_proxy_for_service, do_run, canonicalize_api_names)
from localstack.utils.bootstrap import setup_logging
from localstack.utils.cloudformation import template_deployer
from localstack.services.cloudformation import service_models
from localstack.services.awslambda.lambda_api import BUCKET_MARKER_LOCAL
LOG = logging.getLogger(__name__)
CURRENTLY_UPDATING_RESOURCES = {}
RUN_SERVER_IN_PROCESS = False
MAX_DEPENDENCY_DEPTH = 40
MODEL_MAP = {
    'AWS::StepFunctions::Activity': service_models.StepFunctionsActivity,
    'AWS::SNS::Subscription': service_models.SNSSubscription,
    'AWS::ApiGateway::GatewayResponse': service_models.GatewayResponse,
    'AWS::ApiGateway::Deployment': apigw_models.Deployment,
    'AWS::ApiGateway::Method': apigw_models.Method,
    'AWS::ApiGateway::Resource': apigw_models.Resource,
    'AWS::ApiGateway::RestApi': apigw_models.RestAPI,
    'AWS::StepFunctions::StateMachine': sfn_models.StateMachine,
    'AWS::CloudFormation::Stack': service_models.CloudFormationStack
}
def start_cloudformation(port=None, asynchronous=False, update_listener=None):