ci: use 0 as default NUM_DISKS in minikube.sh
ci: use 0 as default NUM_DISKS in minikube.sh

This is done to prevent conflicts with current ci setup externally
attaching disks.

Signed-off-by: Rakshith R <2c7b05ed1488c7110901626c2c56645675e22767@redhat.com>

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