osd: add --filestore
osd: add --filestore

ceph-disk now defaults to bluestore, which means there wasn't a way to
provision filestore.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@redhat.com>

ceph/ceph-deploy,ceph/ceph-deploy