Tweak usage of Symfony YAML

Instead of using the static method of the main YAML class, I would recommend you to use the dedicated objects directly. This has two main advantages:

* if you are using the registry more than once, it's going to be faster as we don't reinstantiate a parser/dumper each time

* but more important, you probably don't want to support legacy features supported by `Yaml::parse()` (like the possibility to embed PHP in a YAML file or the fact that the input can be a string or a filename).