BackupStatistics: Change the statistics yaml file to a mysql config
The BackupStatistics used to parse a yaml file to connect to mysql. Now, instead of handing mysql credentials as part of the config, convert that into a .ini file, so it can be sent with full flexibility to the mysql connector, and instead remove all the mysql configuration options and merge them into one: stats_file, with the patch of the mysql config file.
This will allow to handle how backups connect to mysql from puppet, including tls options.
Bug: T351491