e2e: use `pstree` instead of `ps` for checking rbd-nbd process
e2e: use `pstree` instead of `ps` for checking rbd-nbd process

The CentOS Stream 8 base container image does not have `ps` installed.
This causes CI jobs to fail, when checking for a restarted rbd-nbd
process.

Instead of using `ps`, the `pstree` command can be used. This will add
some ASCII-tree symbols in front of the command that is logged by the
e2e tests, but that is only used for manual reviewing and does not harm
the running test.

Fixes: #2850
Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi