Add configuration value: compute.default_index_type

[itholic]

related with #722 i added compute.default_index_type

[codecov-io]

Codecov Report

Merging #723 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
+ Coverage   94.18%   94.19%   +<.01%     
==========================================
  Files          32       32              
  Lines        5559     5567       +8     
==========================================
+ Hits         5236     5244       +8     
  Misses        323      323

Impacted Files	Coverage Δ
databricks/koalas/config.py	100% <ø> (ø)	⬆️
databricks/koalas/internal.py	96.29% <66.66%> (ø)	⬆️
databricks/koalas/plot.py	94.91% <0%> (+0.11%)	⬆️

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[HyukjinKwon]

Thanks, merged.