Facultative deployment

It's now possible to define a site in web_php_sites
to create a php-fpm pool, but not to deploy it:
only define 'user' and 'php-fpm' parameters,
but skip 'source' and 'target'.

Environment

A pool can define specific environment variables.
The state now reads the 'env' parameter in the
web_php_sites pillar entry.

This adheres to the 12 factor app environment pattern.

Ensure the php-fpm pool for the user exists