Adopted template overriding from other actions like update

Currently when you use the `SyliusResourceBundle` standalone, when you want to override the template for the delete action you have to define it this way:

``` yaml
my_delete:
    pattern: /my/{id}
    methods: [DELETE]
    defaults:
        _controller: my.controller.my:deleteAction
        template: MyCoreBundle:My:delete.html.twig
```

This is inconsistent with the other actions. This PR changes this so that it works with the folloging syntax:

``` yaml
my_delete:
    pattern: /my/{id}
    methods: [DELETE]
    defaults:
        _controller: my.controller.my:deleteAction
        _sylius:
            template: MyCoreBundle:My:delete.html.twig
```

Before merging this PR we should reconfigure the routes so that `SyliusWebBundle:Backend/Misc:delete.html.twig` will be used again ;-)