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/trunk@1467380 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HDFS-4312-trunk-gd