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. INimbusCredentialPlugin is a plugin interfae that will be executed as part of nimbu's submit topology operation and fetch credential from nimbus process on behalf of user. AutoHDFS will renew the tokens on nimbus host as long as credential has more then 24 hours on renew cycle, if tokens are going to expire in next 24 hours then new tokens will be fetched and pushed to all workers.