call start in sys.exit(start()) instead of start()

Exceptions are raised by the sys.exit() function.
When they are not handled, no stack traceback
is printed in the Python interpreter.
Therefore, when using sys.exit(start())
instead of start() may be more readable and reasonable.

Change-Id: I8887607264c9ead220e66135dab06c9562f0b531

kuryr_libnetwork/server.py

@@ -35,7 +35,7 @@ def start():
 
 
 if __name__ == '__main__':
-    start()
+    sys.exit(start())
 elif 'UWSGI_ORIGINAL_PROC_NAME' in os.environ:
     # The module is being loaded by uWSGI to get the Flask app running under
     # it. This allows Neutron to be set, since uWSGI does not run 'start',