public ClusterManager getClusterManager() {
    return clusterManager;
  }
List of RegionServers killed so far. ServerName also comprises startCode of a server, so any
restarted instances of the same server will have different ServerName and will not coincide
with past dead ones. So there's no need to cleanup this list.
hbase-it/src/test/java/org/apache/hadoop/hbase/DistributedHBaseCluster.java