More safe way to cleanup tombstone nodes

[lucklove]

What problem does this PR solve?

With TiUP, if we want to offline a TiKV server, we need do this first:

tiup cluster scale-in <cluster-name> -N <tikv-node-id>

This will send a request to PD servers to tell them that we want to offline this TiKV server, then PD servers will transfer all regions in this TiKV to some other place. After transferring data, the state of this TiKV server will be tombstone. At this point, the data of this server will be deleted (see the yellow line in the picture below).

But oncel the user execute the display command:

tiup cluster display <cluster-name>

The TiKV server in tombstone state will be destroyed and cleanup at once, this may confuse users and it's unsafe.

What is changed and how it works?

DO NOT cleanup tombstone TiKV servers in display command, but add a prune command to do this, and give the user a choice to cancel the action:

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

[codecov-io]

Codecov Report

Merging #858 into master will decrease coverage by 0.03%.
The diff coverage is 51.54%.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
- Coverage   53.09%   53.06%   -0.04%     
==========================================
  Files         261      263       +2     
  Lines       19014    19038      +24     
==========================================
+ Hits        10096    10102       +6     
- Misses       7358     7370      +12     
- Partials     1560     1566       +6     

Flag	Coverage Δ
#cluster	45.00% <51.02%> (-0.06%)	⬇️
#dm	25.25% <47.16%> (+0.02%)	⬆️
#integrate	47.93% <51.54%> (-0.03%)	⬇️
#playground	22.15% <ø> (-0.02%)	⬇️
#tiup	10.77% <ø> (ø)
#unittest	20.96% <8.33%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cluster/operation/action.go	51.51% <ø> (ø)
components/cluster/command/prune.go	47.61% <47.61%> (ø)
components/dm/command/prune.go	50.00% <50.00%> (ø)
pkg/cluster/manager.go	65.82% <50.00%> (-0.22%)	⬇️
components/cluster/command/display.go	27.41% <100.00%> (-7.03%)	⬇️
components/cluster/command/root.go	45.50% <100.00%> (+0.32%)	⬆️
components/dm/command/display.go	84.61% <100.00%> (+30.91%)	⬆️
components/dm/command/root.go	54.54% <100.00%> (+0.37%)	⬆️
pkg/cluster/operation/scale_in.go	54.00% <100.00%> (ø)
pkg/utils/http_client.go	66.66% <0.00%> (-5.56%)	⬇️
... and 5 more

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[lonng]

LGTM