Correct update-image to image-update for nodepool

Nodepool expects the image updating cron to be called image-update. The
update-image cron that we had ends up being a noop and the default
image-update time is used instead. We want to change the time the image
updating happens which requires we use the correct name.

Change-Id: Ibc84d28705a0bebf95cc105e658941def9108bf7
Merge "Correct update-image to image-update for nodepool"