[feature/crypto] Define default hashing algorithm in service definition file

The default hashing algorithm type is defined as a parameter in the service
definition file for the password hashing system. This will allow us to change
this in the future but it will also prevent unexperienced admins from changing
the hashing algorithm.

PHPBB3-11610
[feature/crypto] Properly set convert_flag and add test for it

PHPBB3-11610