rabase.get_certificate: make serial number arg mandatory
    
    In rabase.get_certificate it does not make sense for the
    serial_number argument to be optional.  Make it a mandatory
    positional argument.
    
    Part of: https://pagure.io/freeipa/issue/3473
    Part of: https://pagure.io/freeipa/issue/5011
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>