In ec2 centOs testing, when `user` is not provided. `AlluxioFuseFileSystem.chown()` will provide a really big number as the `uid: 4294967295`. This uid does not exist in unix and `chown()` will not work.
Need to investigate what uid will be provided to `AlluxioFuseFileSystem.chown()` when user is not provided in `chown and chgrp` in different environment(different linux/mac).
And need to support change group without change user or document this limitation.