ceph-volume: fix regression introcuded via #43536

ceph-volume: fix regression introcuded via #43536

The recent changes from PR #43536 introduced a regeression preventing from
running ceph-volume in a containerized context on Ubuntu 18.04.

Given that the path for the binary `lvs` differs between CentOS 8 and Ubuntu 18.04.
(`/usr/sbin/lvs` and `/sbin/lvs` respictively). It means that ceph-volume running
in the container on CentOS 8 sees the `lvs` binary at `/usr/sbin/lvs` and try to
run it with `nsenter` on the host which is running Ubuntu 18.04.

Fixes: https://tracker.ceph.com/issues/53812
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 95e88cda3df76b59b548ae808df0ef7f19db1f63)
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 3c93ffdc92d4d03b9ae7415b548192a572cfc5ea)