MongoD should use NetworkService account instead of local system account

Our documentation guides our windows users to configure MongoDB to run as a local system account (equivalent of an admin permission on the machine).
http://docs.mongodb.org/manual/tutorial/install-mongodb-on-windows/

In order to maximize the machine security, we should run as NetworkService.
https://msdn.microsoft.com/en-us/library/windows/desktop/ms684272(v=vs.85).aspx