Disable default www pool if not defined in pools attribute.

The package that's installed (at least on Ubuntu) will add `/etc/php5/fpm/pool.d/www.conf`. Thus, even if this pool is not in `node['php-fpm']['pools']`, it will still be enabled after the node converges.

This removes this stock pool unless it is detected in `node['php-fpm']['pools']`.
My original thought when adding the condition was that if the pool was being used, disabling it might cause some unnecessary disruption in service. I'm not sure if that is a real concern though, as in theory the service won't be reloaded until after it is enabled again.