Use sane default XFS options (same as ceph-disk)

This should prevent cluster from being deployed with exotic and potentially
harmful XFS options such as '-n size=64k' which may lead to OSD crashes or even
data corruption.

References:
  - http://lists.ceph.com/pipermail/ceph-users-ceph.com/2014-July/041336.html
  - http://tracker.ceph.com/issues/6301

The new options were taken from ceph upstream recommendation in the ceph-disk
utility.
See https://github.com/ceph/ceph/blob/master/src/ceph-disk#L101: