mon: MonClient: allow pinging a monitor without authenticating first

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
librados: support pinging a monitor without auth via RadosClient

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
pybind: rados: ping a monitor via librados

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
cli: ceph: add support to ping monitors

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>