fix default commit_ts in tiup-cluster

[nexustar]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

• on tiup-cluster before v1.9.x , --initial_commit_ts is set on start script, so it cannot be overwrited be config file.
• on tiup-cluster before v1.8.1, it has wrong default value "0"
• we plan to remove it on start script on tiup-cluster v1.9.0(https://github.com/pingcap/tiup/pull/1706/files) and migrate the configure,please help to review and see if we need to fix wrong default value "0" on old cluster.

2. What did you expect to see?

3. What did you see instead?

4. Please provide the relate downstream type and version of drainer.
   (run drainer -V in terminal to get drainer's version)