HADOOP-9598. Improve code coverage of RMAdminCLI (Aleksey Gorshkov and Andrey Klochkov via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1534906 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-HADOOP-9315-branch-2-gd