ensure /var/log/php-fpm directory exist

Hi, when using ::php::fpm::daemon together with ::php::fpm::conf, it we didn't ensure the /var/log/php-fpm directory, slowlog can't be written into the directory, and therefore php-fpm cannot start.

```
Puppet log:
Debug: /Stage[main]/Php::Fpm::Daemon/File[/var/log/php-fpm]: Nothing to manage: no ensure and the resource doesn't exist

PHP log:
[10-Mar-2014 09:25:59] ERROR: Unable to create or open slowlog(/var/log/php-fpm/www2-slow.log): No such file or directory (2)
```