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Having upgraded to the latest LocalStack version, I've noticed that Docker containers from Lambda executions are immediately removed when they terminate.

This was added in https://github.com/localstack/localstack/pull/1584

I can appreciate the need to clean up containers, however this feature of having them stay up is actually very useful for debugging things when they go wrong!

Would it be possible to have a flag so that when running LocalStack we can specify whether to remove containers on completion? Something like:

```
services:
  localstack:
    image: localstack/localstack:latest
    ports:
      - "4567-4593:4567-4593"
    environment:
      - SERVICES=serverless,ssm,stepfunctions
      - DOCKER_HOST=unix:///var/run/docker.sock
      - LAMBDA_EXECUTOR=docker
      - REMOVE_CONTAINERS_ON_LAMBDA_TERMINATE=true #(or false)
    volumes:
      - "${TMPDIR:-/tmp/localstack}:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
```

Thanks for the great work folks!
import os
import re
import glob
import json
import time
import logging
import threading
import subprocess
import six
from multiprocessing import Process, Queue
try:
    from shlex import quote as cmd_quote
except ImportError:
    from pipes import quote as cmd_quote  
from localstack import config
from localstack.utils.aws import aws_stack
from localstack.utils.common import (