Add zend-opcache module recipe

Also fix a bug when installing PHP: make sure PHP config dir exists
On Centos `conf_dir` is `/etc`, which should exist with the system. On Ubuntu it is owned by `php5-fpm`

> phunehehe@dev2 /home/phunehehe dpkg -S /etc/php5/fpm
> php5-fpm: /etc/php5/fpm

So the fact that the directory does not exist is an error, but it's an error with something else. Maybe `module_opcache` (and `module_apc` and everything else) should depend on `fpm`.