As is specified in 28018, it would be useful to know the manager's addresses
even in a worker node. This is especially useful when there are many
worker nodes in a big cluster.
The information is available in `info.Swarm.RemoteManagers`.
This fix add the information of `Manager Addresses` to the output
of `docker info`, to explicitly show it.
A test has been added for this fix.
This fix fixes 28018.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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