Zookeeper log cleanup

The Zookeeper WAL log files keep growing, so I created a small cron class that only keeps the last 4 versions.

http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html#Ongoing+Data+Directory+Cleanup

The cdh4::zookeeper::cron class should be included on all servers running a zookeeper server. We could  include it in the cdh4::zookeeper::server class, for now it has to be done manually. See the examples.