HDFS-6017. Query the status of rolling upgrade in the preparation stage in TestRollingUpgrade and TestRollingUpgradeRollback. (Contributed by Haohui Mai)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-5535@1571875 13f79535-47bb-0310-9956-ffa450edef68