Using sys.exit(main()) instead of main()

TrivialFix: Similar [1] in Kolla project
As we known, 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(main()) instead of main()
may be more readable and reasonable.

[1] https://review.openstack.org/#/c/349353/

Change-Id: Ie4654b75892de025be73a17a76cdeac1f357ebbd

contrib/lp-cedarish/docker/swift-handler.py

@@ -182,4 +182,4 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
+    sys.exit(main())