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']`.
Perhaps this condition not quite required, since if it there, it will be re-enabled with in the normal flow anyway.
What do you think?