Throw error on unknown keys in cluster.yml

[jakolehm]

[jnummelin]

Not supported out-of-box, but there seem to be some workarounds. They look somewhat dirrty IMO though.

https://github.com/dry-rb/dry-validation/issues/179

[kke]

With Dry::Validation.Form the unknown keys disappear from input before validation

It's quite simple to check for unknown top level keys with:

unless (yaml.keys.map(&:to_sym) - schema_class.rules.keys).empty?
  signal_usage_error ..
end

Anything further than that seem pretty difficult.