Don't cache template if the image already exists

With this change, when an image is initialized it won't try to fetch the
template if the image already exists. We can assume that when the image
already exists on disk there is no need for getting the template because
it is only required when creating the image for the first time.

This also prevents an issue when migrating an instance whose base image
had been deleted or made private for which the migration would fail
while fetching the template image.

Fixes #1206887

Change-Id: Ida95cebf8bad4ab931326d226537cc111820c998