added mapred.system.dir

new PR for https://github.com/wikimedia/operations-puppet-cdh4/pull/4 / https://github.com/wikimedia/operations-puppet-cdh4/issues/3

Forget and close the old PR on the other repo as you are discontinuing it anyway.

Shall I re-PR my other PRs also ?

Change-Id: I9f2aa120d850c44ce53e40ae8538c0d2632ac029
GitHub: https://github.com/wikimedia/puppet-cdh4/pull/17