ceph-config: calculate num_osds for the lvm batch scenario

ceph-config: calculate num_osds for the lvm batch scenario

For now our best guess is to count the number of devices and multiply
by osds_per_device. Ideally we'd like to run ceph-volume lvm batch
--report and get the number of OSDs that way, but currently we need
a ceph.conf in place already before we can do that. There is a tracker
ticket that would allow os to get around the need for a ceph.conf:
http://tracker.ceph.com/issues/36088

Fixes: https://github.com/ceph/ceph-ansible/issues/3135
Signed-off-by: Andrew Schoen <aschoen@redhat.com>