Made terms description nullable in fixtures

As they are nullable as describe in migrations https://github.com/croogo/croogo/blob/master/Plugin/Taxonomy/Config/Migration/1346931401_firstmigrationtaxonomy.php
The form input should use the configured value to tell whether it's multiple or not

Instead of hard coding it to true.
Added useful alt property for images

page_white image didnt have an alt, and thumbnails had generic default alts (thumb); this sets alt for page_white image as the mime type and the title of the attachment for thumbnails.

Signed-off-by: Rachman Chavik <rchavik@xintesa.com>