STORM-346: added AutoHDFS class that will get hdfs delegation tokens on behalf of users, push it to workers and renew the delegation tokens automatically.

Populate credentials is called on the gateway by the end user.  As such there is no reason to use a proxyUser get a delegation token, because I can just get it.  In fact the proxyUser should fail, because regular users are not authorized to act as proxy users.

The idea was to change some of how nimbus used an ICredentialsRenewer, or have a different interface so that nimbus can ask AutoHDFS to populate the credentials at the very beginning. I'll try to explain better in the more general comments.