There appears to be a race condition when multiple threads invoke `GenericProxy.create_ssl_cert` when using `USE_SSL=1`. It causes a cascading failure of the service as shown by the following logs.

```
[36mlocalstack_1                  |[0m 2019-11-27T18:59:49:ERROR:localstack.services.generic_proxy: Exception running proxy on port 4576: [SSL] PEM lib (_ssl.c:3503) Traceback (most recent call last):
[36mlocalstack_1                  |[0m   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 386, in run_cmd
[36mlocalstack_1                  |[0m     server_side=True, certfile=combined_file)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 1158, in wrap_socket
[36mlocalstack_1                  |[0m     ciphers=ciphers)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 750, in __init__
[36mlocalstack_1                  |[0m     self._context.load_cert_chain(certfile, keyfile)
[36mlocalstack_1                  |[0m ssl.SSLError: [SSL] PEM lib (_ssl.c:3503)
[36mlocalstack_1                  |[0m 
[36mlocalstack_1                  |[0m 2019-11-27T18:59:49:ERROR:localstack.services.generic_proxy: Exception running proxy on port 4569: [SSL] PEM lib (_ssl.c:3503) Traceback (most recent call last):
[36mlocalstack_1                  |[0m   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 386, in run_cmd
[36mlocalstack_1                  |[0m     server_side=True, certfile=combined_file)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 1158, in wrap_socket
[36mlocalstack_1                  |[0m     ciphers=ciphers)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 750, in __init__
[36mlocalstack_1                  |[0m     self._context.load_cert_chain(certfile, keyfile)
[36mlocalstack_1                  |[0m ssl.SSLError: [SSL] PEM lib (_ssl.c:3503)
[36mlocalstack_1                  |[0m 
[36mlocalstack_1                  |[0m 2019-11-27T18:59:49:INFO:localstack.multiserver: Starting multi API server process on port 51492
[36mlocalstack_1                  |[0m 2019-11-27T18:59:49:ERROR:localstack.services.generic_proxy: Exception running proxy on port 4568: [SSL] PEM lib (_ssl.c:3503) Traceback (most recent call last):
[36mlocalstack_1                  |[0m   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 386, in run_cmd
[36mlocalstack_1                  |[0m     server_side=True, certfile=combined_file)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 1158, in wrap_socket
[36mlocalstack_1                  |[0m     ciphers=ciphers)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 750, in __init__
[36mlocalstack_1                  |[0m     self._context.load_cert_chain(certfile, keyfile)
[36mlocalstack_1                  |[0m ssl.SSLError: [SSL] PEM lib (_ssl.c:3503)
[36mlocalstack_1                  |[0m 
[36mlocalstack_1                  |[0m 2019-11-27T18:59:49:ERROR:localstack.services.generic_proxy: Exception running proxy on port 4586: [SSL] PEM lib (_ssl.c:3503) Traceback (most recent call last):
[36mlocalstack_1                  |[0m   File "/opt/code/localstack/localstack/services/generic_proxy.py", line 386, in run_cmd
[36mlocalstack_1                  |[0m     server_side=True, certfile=combined_file)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 1158, in wrap_socket
[36mlocalstack_1                  |[0m     ciphers=ciphers)
[36mlocalstack_1                  |[0m   File "/usr/lib/python3.6/ssl.py", line 750, in __init__
[36mlocalstack_1                  |[0m     self._context.load_cert_chain(certfile, keyfile)
[36mlocalstack_1                  |[0m ssl.SSLError: [SSL] PEM lib (_ssl.c:3503)
[36mlocalstack_1                  |[0m 
```

`common.generate_ssl_cert` has some `os.path.exists` checks while later multiple independent `save_file` calls. These look like they might race and cause the 3 ssl files to become invalid. 
import io
import os
import re
import pwd
import grp
import sys
import json
import uuid
import time
import glob
import base64
import socket
import hashlib
import decimal
import logging
import zipfile
import binascii
import calendar
import tempfile
import threading
import subprocess
import six