Fix bug for only one directory in TiFlash storage.* configuration

[JaySon-Huang]

Signed-off-by: JaySon-Huang tshent@qq.com

What problem does this PR solve?

If there is only one directory in TiFlash storage.* configuration, the TiFlashSpec DataDir become unexpected value.
Introduced by #931

What is changed and how it works?

Fix a bug that if there is only one directory in TiFlash storage.* configuration

Check List

Tests

• Unit test

Code changes

• N/A

Side effects

• N/A

Related changes

• Need to cherry-pick to the release branch

Release notes:

NONE

[codecov-io]

Codecov Report

Merging #938 (3f19678) into master (1f0cbac) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
- Coverage   55.70%   55.66%   -0.04%     
==========================================
  Files         263      263              
  Lines       19389    19392       +3     
==========================================
- Hits        10800    10795       -5     
- Misses       6875     6880       +5     
- Partials     1714     1717       +3     

Flag	Coverage Δ
cluster	43.36% <0.00%> (-0.08%)	⬇️
dm	23.99% <0.00%> (+0.05%)	⬆️
integrate	49.90% <0.00%> (-0.05%)	⬇️
playground	20.21% <ø> (ø)
tiup	16.92% <0.00%> (-0.01%)	⬇️
unittest	23.04% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
pkg/cluster/spec/tiflash.go	72.25% <100.00%> (+0.24%)	⬆️
pkg/cluster/spec/util.go	76.74% <0.00%> (-4.66%)	⬇️
pkg/cluster/api/pdapi.go	58.82% <0.00%> (-1.86%)	⬇️
pkg/cluster/manager.go	67.75% <0.00%> (-0.15%)	⬇️
pkg/cluster/api/dmapi.go	60.00% <0.00%> (+1.73%)	⬆️

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[lucklove]

LGTM