[bug] If default templates contain a flaw, auto-registration will fail

If the default templates have a conflict with one another which causes the resulting NetJSON configuration to fail the schema validation, auto-registration will fail with 500 internal server error.

A possible solution would be the following:

• if the current template being saved is flagged as default
• before saving, create a NetJSON configuration from all the default templates using the same backend and try to validate it against the schema used, if the validation fails, trigger a django ValidationError

We must provide a way to extend this behaviour, since downstream apps will need more complex validation rules (openwisp-controller will have to check default templates for each organization if the default template being edited is "shared").

Start by tracing the auto-registration path and the save flow for default templates described in the issue. Verify how the backend builds NetJSON configurations and invokes schema validation, then define completion as raising Django ValidationError before saving invalid default-template combinations while leaving an extension point for downstream validation rules.