When you specify the ID of an image that has a slug, terraform would
store its slug to the state, hence it would always recreate the image.
This commit fixes it by storing the image as an ID when it is specified
by and ID by the user, ignoring the slug.
Closes#12751.
Fixes#12255.
Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>