Dont raise NotFound in simple db image_tag_get_all

This patch removes the check in simple db. This is so both simple db and
sqlalchemy have the same behavior. We do not necessarily want to perform two
queries to see if the image exists so an empty list of tags is returned.

Fixes bug 1197057

Change-Id: Ibae37198b3854aa5dbe0e8a643a3d4b8608666c5
Merge "Dont raise NotFound in simple db image_tag_get_all"