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
YARN-469. Make scheduling mode in FS pluggable. (kkambatl via tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1460962 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-MAPREDUCE-4884-branch-2-gd