sylva-units validation does not prevent typos in unit names

The JSON schema validation of sylva-units does not prevent an error where the user overrides a unit enabled field with a typo in the unit name.

Example:

units:
  monitorig:
    enabled: false

The syntax above will not result in any validation failure because the schema enforces no validation on when enabled: false.

The intent was to facilitate the life of people who use Sylva (e.g not mandate the specification of a workload-cluster kustomization_spec.path when workload-cluster is disabled), but we need to find a better balance.