subtests.docker_cli.save_load: Style cleanups and prep for another subtest

No actual changes, only couple of style cleanups and refactor.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli.save_load: Add stressed_load subsubtest

This subsubtest spawns multiple workers and simultaneously loads
multiple existing and new images.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #284 from ldoktor/save_load

 subtests.docker_cli.save_load: Add stressed_load subsubtest