subtests.docker_cli.save_load: Add stressed_load subsubtest

This pull request refactors the existing test a bit and adds another variant asked by https://github.com/autotest/autotest-docker/issues/248 . I changed the commands to use `sh` instead of `bash` as they don't use anything fancy and `bash` is not present in `busybox` image, which was the only one I could use for testing on my Fedora with `tmpfs` `/tmp` with limited free space...

This subsubtest spawns multiple workers and simultaneously loads
multiple existing and new images.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
In python, because of the GIL most threads end up executing serialy and not very parallel at all.  This is fine for now, but keep this in mind for next time, if you really want it to be parallel, uses processes instead.  There's even a module that provides the same threading.Thread interface, but uses processes (can't remember the name).