do not default fullname to gecos when schema = ad
    
    https://fedorahosted.org/sssd/ticket/1482
    
    When we add fullname to user_attrs, then sysdb_add_basic_user()
    will set fullname to gecos when it initially creates the user
    object in the cache, but it will be overwritten in the same
    transaction when sysdb_store_user() adds all the user_attrs.