Select used parsers in input/output

Usage case
It is api endpoint:
```php
@ApiDoc(output="PathToEntity\EntityWithAssertValidation")
```
NelmioApiDocBundle generated `Return` section:
- `field_1` - parsed by ValidationParser
- `field_2` - parsed by JmsMetadataParser

I expect that section `Return` will contain the actual serialized fields
```php
@ApiDoc(
     output={
          "class"="PathToEntity\EntityWithAssertValidation",
          "parsers"={"Nelmio\ApiDocBundle\Parser\JmsMetadataParser"}
    }
)
```
and the return section will contain only `field_1` - parsed by JmsMetadataParser