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.

I would prefer to see this generated at the startup of nimbus, and stored in the nimbus data structure.  Here every time a topology is submitted we have to create new instances of the plugins and never clean them up.  Also by using the total-storm-conf, it allows the topology to override the list of plugins, potentially loading a class on the classpath that administrators had disabled on purpose.