test: Adjust check-docker-storage for shared root/docker LV on Fedora Atomic
test: Adjust check-docker-storage for shared root/docker LV on Fedora Atomic

Most recent Fedora Atomic now puts the docker storage into the root LV,
instead of an LV of its own: /etc/sysconfig/docker-storage-setup
previously was

    CONTAINER_ROOT_LV_NAME=docker-root-lv
    CONTAINER_ROOT_LV_MOUNT_PATH=/var/lib/docker

and is now

    ROOT_SIZE=+100%FREE

With this, cockpit/docker can still manage and reset the storage pool
(thus `can_manage()` should stay true), and as the docker storage pool
is in a VG, `initially_without_vgroup()` should also stay false.

However, resetting the storage pool now can't guarantee any more that
additional PVs get completely unused, as apparently some files still
land on them which are not docker image/container related. Thus stop the
test before that, as the reminder (ensuring that the added disks are not
in the pool any more, and adding them back) cannot work on Fedora
Atomic.

Add a check that resetting the storage pool actually removes all images
(and that some images exist before), to at least cover that
functionality of pool reset.

Closes #8694

SotolitoLabs/cockpit,mvollmer/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,stefwalter/cockpit,martinpitt/cockpit,martinpitt/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,stefwalter/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,stefwalter/cockpit,mareklibra/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,andreasn/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,cockpit-project/cockpit,andreasn/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,garrett/cockpit,deryni/cockpit,stefwalter/cockpit,mvollmer/cockpit,andreasn/cockpit,cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,andreasn/cockpit,garrett/cockpit,mvollmer/cockpit,mareklibra/cockpit,cockpit-project/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,mareklibra/cockpit,deryni/cockpit,mareklibra/cockpit,mareklibra/cockpit,mareklibra/cockpit,andreasn/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit