[CoreBundle] Enable override of routing config

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I wanted to add the paginatate parameter to the sylius key for the generated routes. But without the change the configuration merge doesn't preserve keys and looked like the following:

```yaml
sylius_core:
    routing:
        %sylius.model.taxon.class%:
            field: permalink
            prefix: /t
            defaults:
                controller: sylius.controller.product:indexByTaxonAction
                sylius:
                    template: SyliusWebBundle:Frontend/Product:indexByTaxon.html.twig
                0:
                    paginate: 9
```

With this change the configs get correctly merged to this:

```yaml
sylius_core:
    routing:
        %sylius.model.taxon.class%:
            field: permalink
            prefix: /t
            defaults:
                controller: sylius.controller.product:indexByTaxonAction
                sylius:
                    template: SyliusWebBundle:Frontend/Product:indexByTaxon.html.twig
                    paginate: 9
```