switch_to_containers: fix umount ceph partitions

switch_to_containers: fix umount ceph partitions

When a container is already running on a non containerized node then the
umount ceph partition task is skipped.
This is due to the container ps command which always returns 0 even if
the filter matches nothing.

We should run the umount task when:
1/ the container command is failing (not installed) : rc != 0
2/ the container command reports running ceph-osd containers : rc == 0

Also we should not fail on the ceph directory listing.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1616159
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit 39cfe0aa65ddd96458ba9d0a031d801efbb0d394)