make the wrapping of addons configurable

When in an horizontal form, you probably do not want to have fields with
addons wrapped in a form-group div, since the negative margins of this
div will make the field looks larger than the other fields.

@Bladrak : this is a first step to solve https://github.com/sonata-project/SonataAdminBundle/issues/2459

I tried it on the sandbox, it works as expected. I did not write any docs, but used an `info()` call in the Configuration class that will make a comment appear in `config:dump-reference SonataCoreBundle`'s output.
@pulzarraider done!

Here is the error message you get when configuring the option with a wrong value : 

>   [Symfony\Component\Config\Definition\Exception\InvalidConfigurationException]                                           
  Invalid configuration for path "sonata_core.form_type": The form_type option value must be one of standard, horizontal  
                                                                                                                          




                                                                  
  >[InvalidArgumentException]                                      
  The form_type option value must be one of standard, horizontal  

Here is the output of `config:dump-reference` : 

```
# Default configuration for "SonataCoreBundle"
sonata_core:

    # Must be one of standard, horizontal
    form_type:            standard
    flashmessage:

        # Prototype
        message:
            css_class:            ~
            types:

                # Prototype
                type:
                    domain:               SonataCoreBundle

```