FPM config refactoring

This PR refactors the complete `php-fpm` config handling. All config parameters were moved from the class `php::fpm` to the new class `php::fpm::config`, which for some reason was a resource previously. It also adds new specs and fixes some random stuff I encountered while crawling through the code base.

Please review if the changes to the config parameters make sense.

:construction: Please don't merge yet as documentation is still missing or not up-to-date. :construction:
i think we should keep `settings` for consistency. @fadenb?