Followup on JSON serialization

[loicmathieu]

Issue description

• Document the BC here: Keep empty object on JSON serialization #2358
• Check if we can get rid of the ObjectMapperFactory now that we didn't customize anything on it and we don't need it to set the ClassLoader
• Check if all plugins that uses Json serialization still works
• Get rid of the remaining NOT_DEFAULT Json inclusion in plugins
  • It is also used in the Template implementation
  • Flow.generateSource may be removed
  • TaskDefault must be computed on the YAML not the serialized form
• Related to TaskDefault: taskDefaults is not working #2260