MasterServices getMaster() {
    return master;
  }
For a given cluster with mixed versions of servers, get a list of servers with lower versions,
where system table regions should not be assigned to. For system table, we must assign regions
to a server with highest version. However, we can disable this exclusion using config:
"hbase.min.version.move.system.tables" if checkForMinVersion is true. Detailed explanation
available with definition of minVersionToMoveSysTables.
@return List of Excluded servers for System table regions.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java