API: add JSON schema validation

[paullatzelsperger]

All APIs should validate incoming request payload against a JSON schema. This is to make sure that

• all required fields are present
• all fields contain allowed values (numbers vs strings)
• no unknown fields are present
• all preconditions are met (e.g. referencing a non-existent foreign object)

[jimmarino]

This is handled by model.Validator. If additional validation is needed that can't be done with that framework, it can be added on a case-by-case basis in the handlers. No Json Schema validation is needed.

Note Schema validation does need to be done for input data in orchestrations.