server: prevent misoperation when there are tiflash nodes (#2242)

[sre-bot]

cherry-pick #2242 to release-3.1

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Signed-off-by: disksing [email protected]

What problem does this PR solve?

Fix #2237

What is changed and how it works?

• When placement rules not enabled, don't allow add tiflash node.
• When the cluster has tiFlash nodes, don't allow disable placement rules.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

[codecov-io]

Codecov Report

Merging #2254 into release-3.1 will increase coverage by 0.14%.
The diff coverage is 89.47%.

@@               Coverage Diff               @@
##           release-3.1    #2254      +/-   ##
===============================================
+ Coverage        74.89%   75.04%   +0.14%     
===============================================
  Files              196      196              
  Lines            20152    20164      +12     
===============================================
+ Hits             15093    15132      +39     
+ Misses            3907     3889      -18     
+ Partials          1152     1143       -9

Impacted Files	Coverage Δ
server/grpc_service.go	58.11% <100%> (+1.89%)	⬆️
server/util.go	54.73% <100%> (+2.51%)	⬆️
server/server.go	65.29% <77.77%> (+1.39%)	⬆️
server/member/leader.go	78.57% <0%> (-1.03%)	⬇️
server/region_syncer/client.go	78.44% <0%> (-0.87%)	⬇️
server/handler.go	51.18% <0%> (-0.44%)	⬇️
server/cluster/cluster.go	80.82% <0%> (-0.35%)	⬇️
server/config/option.go	88.31% <0%> (+0.86%)	⬆️
server/core/storage.go	73.73% <0%> (+1.01%)	⬆️
server/schedulers/adjacent_region.go	76.47% <0%> (+1.06%)	⬆️
... and 7 more

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[rleungx]

/run-all-tests