Add more common ceph options and some smaller fixes

- Added new non-mandatory keys to [global]:
  - osd pool default pg num
  - osd pool default pgp num
  - osd pool default min size
  - osd pool default crush rule
  - cephx require signatures
  - cephx cluster require signatures
  - cephx service require signatures
  - cephx sign messages
- fix documentation for auth_type (none/cephx)
- make osd network addresses non-mandatory