Fix client.TestClientClusterStatus

After HBASE-18511, the master doesn't carry any regions by default. Hence, The active master doesn't report its load because the report thread is sleeping in waitForMasterActive().

  protected void waitForMasterActive(){
    boolean tablesOnMaster = LoadBalancer.isTablesOnMaster(conf);
    while (!(tablesOnMaster && activeMaster) && !isStopped() && !isAborted()) {
      sleeper.sleep();
    }
  }

In conclusion, the ClusterStatue#getServers won't contain the master name.