Fixing the parameter parser (see #1168)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | No yet

Before : 

```yaml
# routing.yml
my_route:
    _sylius:
        criteria:
            enable: $enable
```

The parameter parser returned something like:
```php
array(
    criteria => array(
        'enable' => 'myvalue'
        'parameter_name' => array(
              'enable'
        )
);
```

Now it return two arrays, the names of parameters  are isolated in the second one.