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. 
""" Recursive chmod """
    os.chmod(path, mode)
    for root, dirnames, filenames in os.walk(path):
        for dirname in dirnames:
            os.chmod(os.path.join(root, dirname), mode)
        for filename in filenames:
            os.chmod(os.path.join(root, filename), mode)
def rm_rf(path):
    """
    Recursively removes a file or directory
    """
    if not path or not os.path.exists(path):
        return
    
    chmod_r(path, 0o777)
    
    exists_but_non_dir = os.path.exists(path) and not os.path.isdir(path)
    if os.path.isfile(path) or exists_but_non_dir:
        os.remove(path)
    else:
        shutil.rmtree(path)
def cp_r(src, dst):
    """Recursively copies file/directory"""
    if os.path.isfile(src):
        shutil.copy(src, dst)
    else:
        shutil.copytree(src, dst)
def download(url, path, verify_ssl=True):