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

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.

In this patch we have known main() got return values(eg:0, 1), but it can't
specifies exit status when terminate the main thread by unusing sys.exit().
So when using sys.exit(main()) instead of main() may be more readable and
reasonable.

Change-Id: I6b472e0eb2fceb49bb506785f5188e023e1d3968
(cherry picked from commit 675eb6abaf)

fuel_agent/cmd/agent.py

@@ -147,4 +147,4 @@ def main(actions=None):
 
 
 if __name__ == '__main__':
-    main()
+    sys.exit(main())