Merge pull request #730 from ceph/wip-monc-ping

mon: MonClient: ping monitors without authenticating

* add support on the monitor to reply to MPing messages with the contents of
  'mon_status' and 'health', regardless of a client having authenticated beforehand.

* add support on the MonClient to send a MPing message to a randomly picked
  monitor (it was easier this way, '-m ip:port' allows for targeted ping) and block
  waiting for a reply.

* add support on librados, pybind/rados.py and the 'ceph' tool to send pings to
  monitors.

Resolves: #5984

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>