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. 
out = run('whoami').strip()
    return out == 'root'
def cleanup_resources():
    cleanup_tmp_files()
    cleanup_threads_and_processes()
def generate_ssl_cert(target_file=None, overwrite=False, random=False, return_content=False, serial_number=None):
    
    
    from OpenSSL import crypto
    def all_exist(*files):
        return all([os.path.exists(f) for f in files])
    if target_file and not overwrite and os.path.exists(target_file):
        key_file_name = '%s.key' % target_file
        cert_file_name = '%s.crt' % target_file
        if all_exist(key_file_name, cert_file_name):
            return target_file, cert_file_name, key_file_name
    if random and target_file:
        if '.' in target_file:
            target_file = target_file.replace('.', '.%s.' % short_uid(), 1)
        else:
            target_file = '%s.%s' % (target_file, short_uid())
    
    k = crypto.PKey()