Refactor a bit the php-fpm module

Refactor a bit the php-fpm module

This commit adds a new phpfpm service with a new option that permits to
specify pool-specific php configuration (caveat: now each pool has
distinct php ini file, even if they have the same content)

Make sure that the same php package is used everywhere

Build pdo_mysql using mysql c-connector. Would be good to have the same
with mysqli but it seems not to work