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

I put the format just after system enters master image.
For Android test result will store on random directory after another bug fixed, I didn't handle Android part.

But still have one concern, does it need to umount the test partitions when deploying tarball for Ubuntu desktop image will mount it automatically if the formatting occurs before system enters Gnome.