AMBARI-2313. UI allows adding already existing properties to custom core-site.xml /hdfs-site.xml settings and creates confusion. (Oleg Nechiporenko via yusaku)

git-svn-id: https://svn.apache.org/repos/asf/incubator/ambari/trunk@1490773 13f79535-47bb-0310-9956-ffa450edef68