Don't always disable the default 'www' pool

If using the default 'www' pool, this recipe always disables and re-enables it on every chef-client run. Don't run the `php_fpm_pool` disable statement if the pool explicitly exists in the `node['php-fpm']['pools']` attribute.
ruby provides us with `unless` statement which is a bit more readable and therefore more preferred :)

    unless node['php-fpm']['pools'].key?('www')
      #...
    end