osd: autodiscovery mode, use holders to detect device

osd: autodiscovery mode, use holders to detect device

As reported in
https://github.com/ceph/ceph-ansible/issues/1403 when devices are held
by lvm and `osd_auto_discovery` is set to true, it's not enough to check
for a partition count = 0 since Ansible does not report.
This patch also looks for 'holders' which in a case of lvm corresponds
to the name of the pv. Now we also look for holders = 0.

Fixes: #1403
Signed-off-by: Sébastien Han <seb@redhat.com>