fix #2036 define config keys to allow override

Looks like 'controller' and 'sylius' keys have to be explicitly declared, otherwise the merged config would look like:

```
{
  "defaults": {
    "1": {
      "template": "AcmeBundle:Product:indexByCategorySlug.html.twig"
    },
    "0": "AcmeBundle:Product:indexByCategorySlug",
    "sylius": {
      "template": "SyliusWebBundle:Frontend\\/Product:indexByTaxon.html.twig"
    },
    "controller": "sylius.controller.product:indexByTaxonAction"
  },
  "prefix": "\\/t",
  "field": "permalink"
}
```