tests: use CentOS 8.4 image
tests: use CentOS 8.4 image

CentOS 8.4 vagrant image is available at https://cloud.centos.org
let's use it.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible