Configuration validation error when `sched_options` is set inside a partition

[vkarak]

Although this is the only way to define scheduler options in ReFrame 4.0, this does not work.

How to reproduce

Add the following in one of your partitions:

                    'sched_options': {
                        'use_nodes_options': True
                    }

Then loading this configuration file, you get the following error:

ERROR: failed to load configuration: could not validate configuration files: '['<builtin>', 'config/machines.py']': Additional properties are not allowed ('sched_options' was unexpected)