the master's tiup-cluster display sees two Dashboard url

[WalterWj]

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Here we see that the master's tiup-cluster display sees two Dashboard url(s). It feels a little weird in here.

./bin/tiup-cluster display tidb-xx
Cluster type:       tidb
Cluster name:       tidb-xx
Cluster version: v7.1.5
Deploy user:        tidb
SSH type:           builtin
Dashboard URL: http://172.16.xxx.xx:2399/dashboard
Dashboard URLs: http://172.16.xxx.xx:2399/dashboard

[xhebox]

One will only display dashboard of master PD node, the other will also list dashboard_servers. I'll update thedocument.

[dveeden]

#2502