Fix Instance Image Name cell value

It shows up differently in Project vs Admin table for instance with no image.
Project table shows '-'
Admin table shows '(not found)'
=> Now both tables show '-' as it is consistent with other tables

Also, in the Instance detail page, Image Name shows as a hyperlinked "(not found)"
=> Now the detail page shows a plain text 'None'

Change-Id: Ie865017d4fdd6c5d6e028d9e3bf94dd67c21daca
Closes-Bug: #1277240
Merge "Fix Instance Image Name cell value"