YARN-1182. MiniYARNCluster creates and inits the RM/NM only on start() (Karthik Kambatla via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1532113 13f79535-47bb-0310-9956-ffa450edef68