Changed platform_family? to node['platform']

platform_family? doesn't seem to work in an attributes file, it always entered
the windows path. With node['platform'] the code works as expected.
Changed permissions of wp-config.php

After a new installation the wp-config.php was only readable by it's owner.
Changed the permissions to 0644 so that it is readable by everyone. Otherwise
it would be better to change the files owner to the webserver.