mon: implement --extract-monmap <filename>

This will make for a simpler process for
  http://ceph.com/docs/master/rados/operations/add-or-rm-mons/#removing-monitors-from-an-unhealthy-cluster

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit c0268e27497a4d8228ef54da9d4ca12f3ac1f1bf)