Add PHP-FPM conf and tuning settings

[swalkinshaw]

Allows customization of the PHP-FPM service configuration. Previously only the pool configuration was exposed in Trellis. This allows for more advanced performance tuning.

Note: moved over from #1444 by @paulbrzeski (added you as co-author) to make this easier to manage and deal with conflicts + Git history.

Below is the original description:

Sharing a couple of changes we've made to our Trellis so we can fine tune PHP settings for high traffic websites.

Changes include,

• PHP-FPM crash prevention and optimisation
• Small improvements to make things more semantic
• Various configs to support above

This PR only provides the defaults, you can override these settings like any other using your group_vars configs.

PHP FPM for a server sharing environment with MariaDB.
New variables to add to group_vars/%ENV%/php.yml,

# Allow a max of 100 children, based on 16GB of ram and 164mb per child.
php_fpm_pm_max_children: 100
# Start and go up in small incremenets as we have the CPU to spare.
php_fpm_pm_start_servers: 5
php_fpm_pm_min_spare_servers: 5
php_fpm_pm_max_spare_servers: 35
# Allow lots of requests as we have the resources to spare.
php_fpm_pm_max_requests: 1000

# Emergency restart settings.
# Restart after 60 seconds if 10 PHP instances have not responded for 5 min

# Number of child processes not responding
php_fpm_set_emergency_restart_threshold: true
php_fpm_emergency_restart_threshold: 10

# Length of time children not responding.
php_fpm_set_emergency_restart_interval: true
php_fpm_emergency_restart_interval: 5m

# Time to wait before restarting after interval passes.
php_fpm_set_process_control_timeout: true
php_fpm_process_control_timeout: 60s

More info - https://geekflare.com/php-fpm-optimization/

(previously #1430)