Issue 67 - Add ensure_int function
    
    Bug Description: get_attr_val_int("attr") method raises an Exception if
    the requested attribute does not exist. All other similar methods return
    None in that case.
    
    Fix Description: Add a new ensure_int() utility function that checks
    against None, in consistency with existing functions.
    
    https://pagure.io/lib389/issue/67
    
    Author: Ilias95
    
    Review by: wibrown (Thank you very much!)