Fix DI configuration to allow cache_provider key in cache_driver of type provider

[MortalFlesh]

Hey,

I'm trying to update to version ^2.0 and the installation is failing, because I'm using a cache providers.

My simplified doctrine_cache configuration looks like this:

doctrine_cache:
    providers:
        data: ...

In the version ^1.0 I have orm configuration:

...
    second_level_cache:
        region_cache_driver:
            cache_provider: data
        regions:
            # this region is here because of the bug https://github.com/doctrine/DoctrineBundle/issues/81
            special_region:
                cache_driver:
                    cache_provider: data
...

I figured that it should be like this in version ^2.0

...
    second_level_cache:
        region_cache_driver:
            type: provider
            cache_provider: data
        regions:
            special_region:
                cache_driver:
                    type: provider
                    cache_provider: data
...

BUT this doesn't work, since cache_provider key is not allowed in cache_driver configuration, but it is also required in the https://github.com/doctrine/DoctrineBundle/blob/master/DependencyInjection/DoctrineExtension.php#L736-L738 .

So I've added the scalar_node for cache_provider to fix the situation, I hope it is correct.

I guess for now it could be bypassed by following:

cache_driver:
    type: service
    id: doctrine_cache.providers.data

[alcaeus]

It's not required, this was merely left over from the cleanup. Fact is that using type: provider is no longer supported. DoctrineBundle 2.0 only supports type: pool and type: service.