GH #47: Proposal how to avoid exception logging.

[vadeg]

Fix for #47.

• Added additional check if property or ENV variableCREDENTIALS_DIR exists or not without throwing exception.
• AccessTokensBuilder code refactored. Now exception is not thrown during build if there is no CREDENTIALS_DIR.
• Added test for testing that AccessTokensBuilder receives correct AbstractAccessTokenRefresher

[codecov-io]

Codecov Report

Merging #89 into master will decrease coverage by 0.08%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master      #89      +/-   ##
============================================
- Coverage     72.05%   71.97%   -0.09%     
- Complexity      239      241       +2     
============================================
  Files            48       48              
  Lines           841      842       +1     
  Branches         40       40              
============================================
  Hits            606      606              
  Misses          213      213              
- Partials         22       23       +1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/zalando/stups/tokens/FileSupplier.java	93.75% <80%> (-0.37%)	7 <3> (+1)
.../org/zalando/stups/tokens/AccessTokensBuilder.java	73.43% <85.71%> (-0.38%)	43 <8> (+1)

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[jbellmann]

@vadeg I like it. Thanks for your contributions.