HADOOP-9211. Set default max heap size in HADOOP_CLIENT_OPTS to 512m in order to avoid OOME. Contributed by Plamen Jeliazkov.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1467390 13f79535-47bb-0310-9956-ffa450edef68