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.
This should be named ``sonata_core`` (it should be only one global variable per sonata's bundle)