YARN-1855. Made Application-history server to be optional in MiniYARNCluster and thus avoid the failure of TestRMFailover#testRMWebAppRedirect. Contributed by Zhijie Shen.
svn merge --ignore-ancestry -c 1579838 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1579840 13f79535-47bb-0310-9956-ffa450edef68