cleaning up configuration for sonata

| Q             | A
| ------------- | ---
| Bug fix?      | [yes|no]
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

This enables configuration for form group like the other extensions have, and cleans up the sonata config as its currently very limited.