Fix entity restriction in dropdown

[cedric-anne]

fixes #547

1. the wrong variable was evaluated
2. right: 'all' should be defined only for User dropdown
3. add a entity_sons: true to Entity dropdowns

For Entity dropdown, using entity_sons seems mandatory, otherwise only one value would be proposed. On other fields, maybe we should add a recursive boolean to dropdown in order to giv ability to administrator to define the value to pass to entity_sons.