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. 
return calendar.timegm(timestamp.timetuple())
def mkdir(folder):
    if not os.path.exists(folder):
        try:
            os.makedirs(folder)
        except OSError as err:
            
            if err.errno != 17:
                raise
def ensure_readable(file_path, default_perms=None):
    if default_perms is None:
        default_perms = 0o644
    try:
        with open(file_path, 'rb'):
            pass
    except Exception:
        LOG.info('Updating permissions as file is currently not readable: %s' % file_path)
        os.chmod(file_path, default_perms)
def chown_r(path, user):
    """ Recursive chown """
    uid = pwd.getpwnam(user).pw_uid
    gid = grp.getgrnam(user).gr_gid
    os.chown(path, uid, gid)
    for root, dirs, files in os.walk(path):
        for dirname in dirs:
            os.chown(os.path.join(root, dirname), uid, gid)
        for filename in files:
            os.chown(os.path.join(root, filename), uid, gid)
def chmod_r(path, mode):