Use "docker load" to create "scratch" in hack/make/test-integration-cli (instead of implicitly pulling it from the index)

Creating the "docker save" tarball for "scratch" is pretty simple.  I've also extrapolated the "docker build -t busybox ." logic into a separate "hack/make/.ensure-busybox" file so that it can eventually be reused easier.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)