Validate that 'value' is a positive integer or 0.

def validate_non_negative_integer(option, value):
    """Validate that 'value' is a positive integer or 0.
    """
    val = validate_integer(option, value)
    if val < 0:
        raise ValueError("The value of %s must be "
                         "a non negative integer" % (option,))
    return val