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.

This resolves ticket 5984