There is a MapReduce Client Environment Safety Valve, also known as 'MapReduce Service Advanced Configuration Snippet (Safety Valve) for core-site.xml' found in the gui under mapreduce's configuration ->Service-Wide->Advanced will allow you to add any value that doesn't fit elsewhere. (There is also one for core-site.xml as well.)
Having said that, details can be found on Cloudera's site at: http://www.cloudera.com/content/cloudera/en/documentation/core/latest/topics/cdh_ig_cdh5_install.html