[PHP] Allow to provide a PHP-FPM config file

[lolautruche]

Feature request

The PHP provider embeds a php-fpm.conf but it cannot be tweaked. Just like Nginx config file, it may be useful to alter the config in order to enable/disable a few directives, for advanced use cases.

Motivation

This would definitely be a power user feature, but it could help solving edge cases.

Contribution

See #1168 (I was looking for a way to workaround this issue but couldn't find any way to do so).