default void preMoveTables(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    Set<TableName> tables, String targetGroup) throws IOException {
  }
Called before tables are moved to target region server group
@param ctx         the environment to interact with the framework and master
@param tables      set of tables to move
@param targetGroup name of group
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java