Merge -c 1460954 from trunk to branch-2 for YARN-498. Unmanaged AM launcher does not set various constants in env for an AM, also does not handle failed AMs properly (Hitesh Shah via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1460955 13f79535-47bb-0310-9956-ffa450edef68