Add custom config file validation tool

[schaefi]

If a custom config file is used but contains invalid or wrong data due to the schema this will cause the migration to fail. Users will realize that on reboot or on call of run-migration depending on which method they prefer to start the migration.

It would be good to have a tool that is installed with the migration rpm package which allows to validate an eventually existing custom config file prior the actual start of the migration.

The tool should also be integrated into the run-migration code such that users who choose this startup method will automatically get their config file validated