Fixes #3958 : Add migration script to add comment to rudder.batch.reportscleaner.frequency property

[VinceMacBuche]

http://www.rudder-project.org/redmine/issues/3958

[jooooooon]

Thanks for doing this Vincent, but I think it is not necessary. Adding a 4-5 line migration script to add a comment in the config file is overly zealous, and actually introduce more risk of affecting this version's stability than not doing it - it a bug is in the migration script (I'm not saying there is one, just that it could easily happen) we would future upgrades, and adding a comment is not a valid reason to do this.

I'm happy to just have this comment added to new installations of Rudder.