cephadm-adopt: bindmount /var/lib/ceph with 'ro'

cephadm-adopt: bindmount /var/lib/ceph with 'ro'

When collocating osds with iscsigw daemons, cephadm bindmounts the
following:

```
-v /var/lib/ceph/6126c064-6a9e-4092-8a64-977930df0843/iscsi.rbd.ceph-ameenasuhani-4fs3bq-node5.vomtqb/configfs:/sys/kernel/config
```

this prevents cephadm-adopt playbook from running container and bindmounting `/var/lib/ceph:/var/lib/ceph:z`

since 'ro' is enough in this playbook, let's replace the ':z' option on
this bindmount with ':ro'

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2027411
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>