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.

Why does the plugin need to be cleaned up as soon as the submission is over?  Creating an object each time a method is called feels like a lot of overhead to me.  It also makes it so that the plugin does not have the option to cache things and amortize the overhead across multiple calls.  Not that the code currently needs to worry about it, as the current code is more or less just a static function.

> If we generate these instances at startup then it means anyone wanting to use it will have to change the nimbus config and restart nimbus.

Yes, I can see that.  But that is kind of what I want.  It becomes a balancing  act between convenience/insecurity and security/inconvenience.  I just feel nervous that anything on the classpath the user gets to decide to load it into memory. It feels like it is too open ended, but honestly if you feel strongly about it I will not push it.  I would just want the config changed so that it is prefixed with topology instead of nimbus so that it is obvious that it is a topology specific config.