[FrameworkBundle] Fixed validator factory definition when the Validator API is "auto" for PHP < 5.3.9

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11580
| License       | MIT
| Doc PR        | -
That didn't work. The problem is that the callbacks (for normalization, validation etc.) are only executed when a value is set (in the config), but *not* when it is set to its default value. For this reason I ended up adding the callback to the root node, where it is always executed.

I used a `beforeNormalization` closure first, but there the problem is that the "validation" node has not yet been processed. If, for example, the "validation" key does not exist in the config and I set it in the callback, the configuration will later assume that validation is enabled (since the "validation" key is set by the callback), and not that it is disabled (since the "validation" key is *not set* by the config). Therefore I ended up using a `validate` closure, which is executed at the very end of the config normalization.

The `auto` value does not exist anymore. If you set `api`, you need to set it to either `2.4`, `2.5` or `2.5-bc`. If you want the `api` to be chosen automatically, remove it from the configuration.