Return 204 when image data does not exist

Currently a GET on image-data in glance v2 returns 404 when data does not
exist. But according to the API docs the HTTP status code returned in this
case should be 204.

This fix makes Glance return the right HTTP status code.

Change-Id: I34cf68e96f631f05cff349329245f74aee42397c
Closes-Bug: #1251055
Merge "Return 204 when image data does not exist"