cluster: allow scale-in when there is no tispark master node

[AstroProfundis]

What problem does this PR solve?

Fix #1354

What is changed and how it works?

It's reasonable to scale in the remaining tispark worker node in such situation

Check List

Related changes

• Need to cherry-pick to the release branch

Release notes:

cluster: allow scale-in when there is no tispark master node

[codecov-commenter]

Codecov Report

Merging #1363 (6307a43) into master (b1b03c9) will increase coverage by 29.05%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           master    #1363       +/-   ##
===========================================
+ Coverage   25.38%   54.43%   +29.05%     
===========================================
  Files         265      290       +25     
  Lines       20462    22346     +1884     
===========================================
+ Hits         5194    12165     +6971     
+ Misses      14471     8275     -6196     
- Partials      797     1906     +1109     

Flag	Coverage Δ
cluster	41.81% <75.00%> (?)
dm	24.76% <75.00%> (?)
integrate	48.98% <75.00%> (+33.46%)	⬆️
playground	13.78% <ø> (?)
tiup	15.52% <ø> (ø)
unittest	21.68% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
pkg/cluster/manager/scale_in.go	58.69% <75.00%> (+58.69%)	⬆️
components/playground/instance/ticdc.go	0.00% <0.00%> (ø)
pkg/cluster/operation/operation.go	78.26% <0.00%> (ø)
pkg/logger/audit.go	53.33% <0.00%> (ø)
pkg/cluster/operation/destroy.go	56.61% <0.00%> (ø)
components/playground/instance/tiflash_config.go	100.00% <0.00%> (ø)
components/playground/instance/tikv.go	67.34% <0.00%> (ø)
pkg/cluster/operation/check.go	46.70% <0.00%> (ø)
...onents/playground/instance/tiflash_proxy_config.go	81.81% <0.00%> (ø)
components/playground/instance/tidb.go	81.25% <0.00%> (ø)
... and 210 more

---

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[lucklove]

/lgtm

[ti-chi-bot]

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• lucklove

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[lucklove]

/merge

[ti-chi-bot]

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

Commit hash: 6307a43