cluster: support init root password

[AstroProfundis]

What problem does this PR solve?

Close #1660

What is changed and how it works?

An --init argument is added to tiup-cluster start subcommand, it generates a 18 chars long random password and set it to the root@'%' user.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Related changes

• Need to update the documentation

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nexustar

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[nexustar]

Is it available for TLS-enabled cluster？

[codecov-commenter]

Codecov Report

Merging #1700 (3e9e141) into master (26e5521) will increase coverage by 5.86%.
The diff coverage is 36.59%.

@@            Coverage Diff             @@
##           master    #1700      +/-   ##
==========================================
+ Coverage   47.33%   53.19%   +5.86%     
==========================================
  Files         301      303       +2     
  Lines       34011    34096      +85     
==========================================
+ Hits        16098    18135    +2037     
+ Misses      15797    13827    -1970     
- Partials     2116     2134      +18     

Flag	Coverage Δ
cluster	41.39% <18.29%> (+0.55%)	⬆️
dm	24.78% <0.00%> (-0.02%)	⬇️
playground	12.76% <0.00%> (-0.02%)	⬇️
tiup	14.64% <0.00%> (+<0.01%)	⬆️
unittest	21.37% <21.95%> (∅)

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

Impacted Files	Coverage Δ
components/cluster/command/test.go	50.00% <ø> (-2.75%)	⬇️
components/cluster/command/start.go	35.37% <17.54%> (-42.41%)	⬇️
pkg/crypto/rand/passwd.go	75.00% <75.00%> (ø)
pkg/cluster/spec/tidb.go	66.19% <100.00%> (+0.25%)	⬆️
pkg/cluster/template/scripts/tidb.go	78.18% <100.00%> (+1.26%)	⬆️
pkg/queue/any_queue.go	0.00% <0.00%> (ø)
pkg/telemetry/report.pb.go	1.06% <0.00%> (+0.03%)	⬆️
pkg/cluster/manager/builder.go	71.94% <0.00%> (+0.65%)	⬆️
pkg/cluster/operation/action.go	65.16% <0.00%> (+0.75%)	⬆️
pkg/cluster/manager/display.go	47.70% <0.00%> (+0.92%)	⬆️
... and 61 more

---

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[AstroProfundis]

Is it available for TLS-enabled cluster？

Yes, as we don't support TLS encrypted MySQL client yet, the protocol between client and TiDB server is identical with those TLS disabled.

[srstack]

Is it available for TLS-enabled cluster？

TLS-enabled means to enable TLS encrypted communication between TiDB components, but does not include the communication between MySQL Client and TiDB.

[srstack]

/LGTM

[ti-chi-bot]

@srstack: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

In response to this:

/LGTM

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[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: 3e9e141