Provision php-fpm pools to legacy web server

Summary:
Support php-fpm processes, to be able to offer several PHP versions.

Each site receives an ondemand pool.

Ref T1268

Test Plan: service php-fpm status

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1268

Differential Revision: https://devcentral.nasqueron.org/D1249