[feature/passwords] Use common salt generation function for all types

We will be using the same method inside the driver helper class for all hash
types. This is the same function that has been used for the salted md5 hash
of phpBB 3.0.

PHPBB3-11610

phpBB/includes/crypto/driver/bcrypt.php

@@ -81,6 +81,6 @@ class phpbb_crypto_driver_bcrypt extends phpbb_crypto_driver_base
 	*/
 	protected function get_random_salt()
 	{
-		return substr(str_replace('+', '.', bin2hex(openssl_random_pseudo_bytes(22))), 0, 22);
+		return $this->helper->hash_encode64($this->helper->get_random_salt(22), 22);
 	}
 }