Put any custom setting in the right section

We were putting global custom settings in [osd] and re-creating a
[osd]/[mon] section if there were custom settings.
Move "osd pool default" settings to global section in ceph.conf

That's where they belong.
Set reasonable values for some settings

- default pool size: number of OSD nodes, or 3 if more OSDs than that
- default pg and pgp: use algorithm mentioned at
  http://ceph.com/docs/master/rados/configuration/pool-pg-config-ref/
- crush chooseleaf type: use value as mentioned at
  http://ceph.com/docs/master/rados/configuration/ceph-conf/#example-ceph-conf

Fix https://bugzilla.novell.com/show_bug.cgi?id=887696
Merge pull request #69 from vuntz/fix-ceph.conf

Set reasonable values for some settings