cluster: add envs to skip topology sanity check in scale-in process

[AstroProfundis]

What problem does this PR solve?

Close #1616

What is changed and how it works?

Add 2 new environment variables in pkg/cluster/operation:

• SKIP_SCALEIN_TOPO_CHECK: If not empty, skip topology sanity check so that it's possible to delete the last PD, the last TiKV and the last TiSpark master node with worker node remains
• FORCE_PD_ENDPOINTS: A common separated list of available PD endpoints. Used to overwrite pd list in the topology, so that when there is no PD available in the topology tiup-cluster acknowledged but some are working in the actual cluster, it's still possible to scale in other instances

These two environment are very dangerous and are highly possible to make the cluster broken, so they are not supposed to be documented in any public documentations, they are only supposed to be used by developers and expert users who know 100% what they are doing.

Check List

Tests

• Not tested, use as your own risk...

Code changes

• Has exported function/method change

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• nexustar

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[codecov-commenter]

Codecov Report

Merging #1627 (4c6a6a7) into master (d4de166) will increase coverage by 25.10%.
The diff coverage is 27.65%.

❗ Current head 4c6a6a7 differs from pull request most recent head de2315b. Consider uploading reports for the commit de2315b to get more accurate results

@@             Coverage Diff             @@
##           master    #1627       +/-   ##
===========================================
+ Coverage   15.59%   40.70%   +25.10%     
===========================================
  Files         151      260      +109     
  Lines       15234    23349     +8115     
===========================================
+ Hits         2376     9504     +7128     
- Misses      12341    12457      +116     
- Partials      517     1388      +871     

Flag	Coverage Δ
cluster	37.30% <27.65%> (?)
dm	26.13% <4.25%> (?)
integrate	40.70% <27.65%> (+25.10%)	⬆️
tiup	?
unittest	?

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

Impacted Files	Coverage Δ
pkg/cluster/operation/operation.go	71.42% <ø> (ø)
pkg/cluster/operation/scale_in.go	51.32% <18.75%> (ø)
pkg/cluster/operation/destroy.go	52.75% <42.85%> (ø)
pkg/cluster/operation/upgrade.go	58.53% <50.00%> (ø)
pkg/repository/store/store.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/repository/utils/hash.go	0.00% <0.00%> (-83.34%)	⬇️
pkg/repository/model/publish.go	0.00% <0.00%> (-83.34%)	⬇️
pkg/repository/store/local.go	0.00% <0.00%> (-82.61%)	⬇️
pkg/repository/progress.go	23.07% <0.00%> (-76.93%)	⬇️
pkg/telemetry/meta.go	0.00% <0.00%> (-67.40%)	⬇️
... and 247 more

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[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 533a951