trying one by one.
In production, monitors may crash (or have a network problem), if we try
monitors one by one, rbd command will hang a long time (e.g. `rbd map -m
<unconnectable_host_ip>` on linux 4.4 timed out in 6 minutes) when
trying a unconnectable monitor. This is unacceptable.
Actually, we can simply pass a comma-separed list monitor addresses
to `rbd` command utility. Kernel rbd/libceph modules will pick
monitor randomly and try one by one, `rbd` command utility succeed soon
if there is a good one in monitors list.
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