Avoid errors on some actions when image not usable

Using the metadata saved on instance creation, we can now get all the
image related metadata we need from the instance itself.

This patch replace the logic for getting the image metadata on some
actions that shouldn't fail when the image is not accessible (create
an snapshot, resize, migrate, rescue an instance or attach an
interface).

Fixes bug 1039662

Change-Id: Id8c2f13ca96908317d271a84fd685104b64f87e9