YARN-1855. Made Application-history server to be optional in MiniYARNCluster and thus avoid the failure of TestRMFailover#testRMWebAppRedirect. Contributed by Zhijie Shen.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1579838 13f79535-47bb-0310-9956-ffa450edef68