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