cluster: refine log_dir handling on clean (part 1)

[AstroProfundis]

What problem does this PR solve?

Part of #947

What is changed and how it works?

• Mark experimental features more visible
• Check for paths to avoid deleting files if data dir is a sub path of log dir
• Only delete *.log when deleting log files

Check List

Tests

• Manual test (add detailed scripts or steps below)

Code changes

• Has persistent data change

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release notes:

cluster: adjust checking process when cleaning log files

[codecov-io]

Codecov Report

Merging #981 (f33a73d) into master (01bf92c) will decrease coverage by 25.46%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##           master     #981       +/-   ##
===========================================
- Coverage   55.40%   29.93%   -25.47%     
===========================================
  Files         279      265       -14     
  Lines       19709    18358     -1351     
===========================================
- Hits        10919     5495     -5424     
- Misses       7065    12014     +4949     
+ Partials     1725      849      -876     

Flag	Coverage Δ
cluster	?
dm	?
integrate	21.52% <0.00%> (-28.14%)	⬇️
playground	20.28% <0.00%> (-0.01%)	⬇️
tiup	16.45% <0.00%> (-0.01%)	⬇️
unittest	23.10% <85.71%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
pkg/set/string_set.go	50.00% <0.00%> (-22.23%)	⬇️
components/cluster/command/clean.go	64.51% <100.00%> (-21.70%)	⬇️
components/cluster/command/deploy.go	32.50% <100.00%> (-35.00%)	⬇️
components/cluster/command/root.go	26.50% <100.00%> (-19.28%)	⬇️
components/cluster/command/scale_out.go	30.55% <100.00%> (-58.34%)	⬇️
components/dm/command/root.go	31.96% <100.00%> (-23.04%)	⬇️
pkg/meta/paths.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/utils/utils.go	0.00% <0.00%> (-100.00%)	⬇️
components/dm/main.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/logger/log/log.go	0.00% <0.00%> (-100.00%)	⬇️
... and 178 more

---

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[lucklove]

/lgtm

[lucklove]

/merge

[ti-chi-bot]

@lucklove: adding 'status/can-merge' to this PR must have 1 LGTMs

In response to this:

/merge

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[lucklove]

/lgtm

[lucklove]

/merge

[ti-chi-bot]

Can merge label has been added.

Git tree hash: f33a73d

[ti-chi-bot]

@AstroProfundis: Your PR has out-of-dated and I have automatically updated it for you.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the tidb-community-bots/ti-community-prow repository.