Adding support for HDFS proxy

Currently, HDFS doesn't authenticate users. The client simply tells HDFS who she is and HDFS will take her word for it. That makes it possible for HDFS proxy to access HDFS on behalf of a user - the proxy simply claims to be that user. Once we turn on authentication, the proxy can't do that without having user's credentials. We need a solution such that HDFS proxy can continue to access HDFS on behalf of users.