Hey, first thanks for this amazing project!

I was trying to use serverless and localstack together, and I was having problems deploying a lambda function. In the logs I was getting errors 502 all the time:

```Serverless: Recoverable error occurred (502), sleeping for 5 seconds. Try 4 of 4

  Serverless Error ---------------------------------------

  502
```

I then start tracking the error, since this one above didn't give me much information, and I ended up at Serverless calling cloud formation's `describe-stack-events` using the unique stack id, instead of the stack name:

https://github.com/serverless/serverless/blob/f93b27bf684d9a14b1e67ec554a7719ca3628135/lib/plugins/aws/lib/monitorStack.js#L33

That is a legit request, as seen in the aws docs:

https://docs.aws.amazon.com/cli/latest/reference/cloudformation/describe-stack-events.html#options

The cloudformation mock within localstack accepts the stackName without problems:

```
> aws --endpoint-url=http://localhost:4581 cloudformation list-stacks    
{
    "StackSummaries": [
        {
            "StackId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "StackName": "steamRoller-local",
            "CreationTime": "2011-05-23T15:47:44Z",
            "StackStatus": "UPDATE_COMPLETE",
            "TemplateDescription": "The AWS CloudFormation template for this Serverless application"
        }
    ]
}

> aws --endpoint-url=http://localhost:4581 cloudformation describe-stack-events --stack-name steamRoller-local
{
    "StackEvents": [
        {
            "StackId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "EventId": "a49fd67b-ce7b-4a33-9a00-6ebe2242adae",
            "ResourceStatus": "UPDATE_COMPLETE",
            "ResourceType": "AWS::CloudFormation::Stack",
            "Timestamp": "2019-08-10T13:44:44.550022Z",
            "StackName": "steamRoller-local",
            "PhysicalResourceId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "StackName": "steamRoller-local",
            "PhysicalResourceId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "LogicalResourceId": "steamRoller-local"
        },
        {
            "StackId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "EventId": "35ef5c91-99a0-483d-b6b7-e01ee3dda2dd",
            "ResourceStatus": "CREATE_IN_PROGRESS",
            "ResourceType": "AWS::CloudFormation::Stack",
            "Timestamp": "2019-08-10T13:37:51.124574Z",
            "ResourceStatusReason": "User Initiated",
            "StackName": "steamRoller-local",
            "PhysicalResourceId": "arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3",
            "LogicalResourceId": "steamRoller-local"
        }
    ]
}
```

But it doesn't accept the stackId as the argument (the one used by serverless):

```
> aws --endpoint-url=http://localhost:4581 cloudformation describe-stack-events --stack-name arn:aws:cloudformation:us-east-1:000000000000:stack/steamRoller-local/fcbeb0ef-9739-44a2-8a37-f599bbf4e5f3

An error occurred (502) when calling the DescribeStackEvents operation (reached max retries: 4): Bad Gateway
```

If I have a look at the docker logs I can see:

```
"invalid XML received:\n%s" % (e, xml_string)) botocore.parsers.ResponseParserError: Unable to parse response (syntax error: line 1, column 54), invalid XML received:

b'<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">\n<title>500 Internal Server Error</title>\n<h1>Internal Server Error</h1>\n<p>The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.</p>\n'
```

That matches what other colleagues were seeing: https://github.com/localstack/localstack/issues/965

Please, make localstack's cloudformation handle the stackId in describe-stack-events and other actions that accepts it. This is a blocker for using localstack for me :/

Thanks
import re
import os
import imp
import sys
import json
import uuid
import time
import base64
import logging
import zipfile
import threading
import traceback
import hashlib
from io import BytesIO
from datetime import datetime
from six.moves import cStringIO as StringIO
from six.moves.urllib.parse import urlparse
from flask import Flask, Response, jsonify, request
from localstack import config
from localstack.constants import TEST_AWS_ACCOUNT_ID
from localstack.services import generic_proxy