Allow environment variables when configuring FPM pools

[chrisboulton]

Pull Request (PR) description

Allows for PHP FPM pool settings to be configured as environment variables (pm_max_children => '${PM_MAX_CHILDREN}') in addition to their existing typed formats.

I debated about the best way to add this, and ended up adding Variant types, along with a single custom type. I did this because there were varying types (Integer[0], Integer[1], Php::Duration) that I didn't want to have to duplicate.

The env based configuration applies to every setting, but for right now I've added it to the most common ones where due to the stricter validation, it would previously fail.

This Pull Request (PR) fixes the following issues

Fixes #649