ceph-validate: use root device from ansible_mounts
ceph-validate: use root device from ansible_mounts

Instead of using findmnt command to find the device associated to the
root mount point then we can use the ansible_mounts fact.

Signed-off-by: Dimitri Savineau <a50a620e679eed14f4e99998b821cea8884cf228@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible