Code review comment for lp://qastaging/~qzhang/lava-dispatcher/fix-867858

I'm not sure it wouldn't be safer to just make sure the first thing we do before trying to deploy an image, is to reformat the partitions. There's a window of opportunity, where you could have a failure before the image deploy step gets to the point of formatting the partitions (for instance, if there's a failure during l-m-c). If anything is leftover from the previous run, then we could pick it up accidentally.