Specify postpone-cut-over-flag-file, but the file did't created

I don't know why not /tmp/ghost.postpone.flag was not created

```
2018-02-06 06:10:39 ERROR Maximal timeout is 10sec. Timeout remains at 3
2018-02-06 06:10:39 INFO starting gh-ost 1.0.42
2018-02-06 06:10:39 INFO Migrating `elite`.`dn_status_detail`
2018-02-06 06:10:39 INFO connection validated on 127.0.0.1:3306
2018-02-06 06:10:39 INFO User has ALL privileges
2018-02-06 06:10:39 INFO binary logs validated on 127.0.0.1:3306
2018-02-06 06:10:39 INFO Restarting replication on 127.0.0.1:3306 to make sure binlog settings apply to replication thread
2018-02-06 06:10:40 INFO Inspector initiated on Mysql-mdb03a:3306, version 5.6.21-log
2018-02-06 06:10:40 INFO Table found. Engine=InnoDB
2018-02-06 06:10:40 INFO Estimated number of rows via EXPLAIN: 22997270
2018-02-06 06:10:40 INFO Recursively searching for replication master
2018-02-06 06:10:41 INFO Master found to be 10.101.129.18:3306
2018-02-06 06:10:41 INFO log_slave_updates validated on 127.0.0.1:3306
2018-02-06 06:10:41 INFO connection validated on 127.0.0.1:3306
2018/02/06 06:10:41 binlogsyncer.go:75: ESC[0;37m[info] create BinlogSyncer with config &{99999 mysql 127.0.0.1 3306 root ***** false false }ESC[0m
2018-02-06 06:10:41 INFO Connecting binlog streamer at mysql-bin.000027:198936592
2018/02/06 06:10:41 binlogsyncer.go:241: ESC[0;37m[info] begin to sync binlog from position (mysql-bin.000027, 198936592)ESC[0m
2018/02/06 06:10:41 binlogsyncer.go:134: ESC[0;37m[info] register slave for master server 127.0.0.1:3306ESC[0m
2018/02/06 06:10:41 binlogsyncer.go:568: ESC[0;37m[info] rotate to (mysql-bin.000027, 198936592)ESC[0m
2018-02-06 06:10:41 INFO rotate to next log name: mysql-bin.000027
2018-02-06 06:10:41 INFO connection validated on 10.101.129.18:3306
2018-02-06 06:10:41 INFO connection validated on 10.101.129.18:3306
2018-02-06 06:10:41 INFO will use time_zone='SYSTEM' on applier
2018-02-06 06:10:41 INFO Examining table structure on applier
2018-02-06 06:10:41 INFO Applier initiated on Mysql-mdb03b:3306, version 5.6.21-log
2018-02-06 06:10:41 INFO Droppping table `elite`.`_dn_status_detail_ghc`
2018-02-06 06:10:41 INFO Table dropped
2018-02-06 06:10:41 INFO Creating changelog table `elite`.`_dn_status_detail_ghc`
2018-02-06 06:10:41 INFO Changelog table created
2018-02-06 06:10:41 INFO Creating ghost table `elite`.`_dn_status_detail_gho`
2018-02-06 06:10:41 INFO Ghost table created
2018-02-06 06:10:41 INFO Altering ghost table `elite`.`_dn_status_detail_gho`
2018-02-06 06:10:41 INFO Ghost table altered
2018-02-06 06:10:41 INFO Waiting for ghost table to be migrated. Current lag is 0s
2018-02-06 06:10:41 INFO Intercepted changelog state GhostTableMigrated
2018-02-06 06:10:41 INFO Handled changelog state GhostTableMigrated
2018-02-06 06:10:41 INFO Chosen shared unique key is PRIMARY
2018-02-06 06:10:41 INFO Shared columns are REC_GUID,DeviceDN,DevicePID,Status,StartTime,EndTime,ConnectionID,Queue,OtherDN,OriginalANI,OriginalDNIS,CallType,NEXTSTATUS,AgentID,Duration,agenttoagent,LASTSTATUS,vCID,lastredirectingdevice
2018-02-06 06:10:41 INFO Listening on unix socket file: /tmp/gh-ost.elite.dn_status_detail.sock
2018-02-06 06:10:41 INFO As instructed, counting rows in the background; meanwhile I will use an estimated count, and will update it later on
2018-02-06 06:10:41 INFO As instructed, I'm issuing a SELECT COUNT(*) on the table. This may take a while
2018-02-06 06:10:41 INFO Migration min values: [00000003-a8e9-4b7e-9701-210be391acfc]
2018-02-06 06:10:41 INFO Migration max values: [ffffffdc-02ef-412a-8d66-ec85a3bfb481]
2018-02-06 06:10:41 INFO Waiting for first throttle metrics to be collected
2018-02-06 06:10:41 INFO First throttle metrics collected
```

```
# Migrating `elite`.`dn_status_detail`; Ghost table is `elite`.`_dn_status_detail_gho`
# Migrating Mysql-mdb03b:3306; inspecting Mysql-mdb03a:3306; executing on Mysql-mdb03a
# Migration started at Tue Feb 06 06:10:39 +0800 2018
# chunk-size: 1000; max-lag-millis: 1500ms; dml-batch-size: 10; max-load: Threads_running=300; critical-load: Threads_running=300; nice-ratio: 0.000000
# throttle-additional-flag-file: /tmp/gh-ost.throttle
# throttle-control-replicas count: 1
# postpone-cut-over-flag-file: /tmp/ghost.postpone.flag
# panic-flag-file: /tmp/ghost.panic.flag
# Serving on unix socket: /tmp/gh-ost.elite.dn_status_detail.sock
```

detail log
[dn_status_detail.log](https://github.com/github/gh-ost/files/1705557/dn_status_detail.log)

Thank you!