replace function _prepare_dirs() with smart properties

[funbringer]

Actually, it's a bunch of fixes and improvements.

I decided to move code from _prepare_dirs() into several properties. Note that PostgresNode.base_dir property will not automatically call initdb if there's no such dir.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   97.17%   97.02%   -0.15%     
==========================================
  Files          16       16              
  Lines        1239     1244       +5     
==========================================
+ Hits         1204     1207       +3     
- Misses         35       37       +2

Impacted Files	Coverage Δ
testgres/api.py	100% <100%> (ø)	⬆️
testgres/consts.py	100% <100%> (ø)	⬆️
testgres/node.py	97.26% <100%> (+0.02%)	⬆️
testgres/utils.py	94.89% <0%> (-2.05%)	⬇️

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