[COOK-2510] Change app_root to mod_php_apache2 resource attribute.

In http://tickets.opscode.com/browse/COOK-2395 the application_php cookbook was updated to move the app_root attribute out of the application cookbook and into the application_php cookbook. However, the attribute was added to the php resource instead of the mod_php_apache2 resource which calls app_root, this results in a 
```
FATAL: NoMethodError: undefined method `app_root' for Chef::Resource::ApplicationPhpModPhpApache2
```
 
whenever the cookbook is run with app_root defined.

The fix is to move the app_root attribute into the mod_php_apache2 resource instead of php.