allow to choose the "random source" when generating scoped keys

Whene generating a key with mcrypt_create_iv(), it can fail depending on the system you are on (I got several timeout while trying to generate scoped keys on my DEV and PROD Debian servers and in a Linux VM, but none on Windows).
This will allow users to choose what kind of random source works the best for them.
For more information : http://php.net/manual/en/function.mcrypt-create-iv.php