Harmonize policy discovery to kdb driver
    
    The KDB driver does not walk the tree back like the original password plugin.
    Also we do not store the default policy in the base DN as we used to do in the
    past anymore.
    So doing a full subtree search and walking back the tree is just a waste of
    time.
    Instead hardcode the default policy like we do in the kdb driver.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/4085