tests/library: define container cmd once
tests/library: define container cmd once

In containerized deployment, the ceph_volume module will always uses
the same container command prefix for all actions.
Instead of duplicate this code in all container tests we can define it
once.

Signed-off-by: Dimitri Savineau <a50a620e679eed14f4e99998b821cea8884cf228@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible