tests: set `volume_cache` 'unsafe'
tests: set `volume_cache` 'unsafe'

Set volume_cache to unsafe for CI VMs.
We might be using tmpfs for volume disks soon, therefore 'unsafe' is a
prerequisite for volume_cache

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

travmi/ceph-ansible,font/ceph-ansible,fgal/ceph-ansible,jtaleric/ceph-ansible,ceph/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible,jtaleric/ceph-ansible,ceph/ceph-ansible,travmi/ceph-ansible