Fix schema validation for templates

[bkryza]

Fixes #361

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.41%. Comparing base (d3f9101) to head (ef84f61).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/config/yaml_decoders.cc	81.25%	3 Missing ⚠️
src/cli/cli_handler.cc	92.85%	2 Missing ⚠️
src/config/yaml_emitters.cc	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   90.25%   90.41%   +0.16%     
==========================================
  Files         151      151              
  Lines       18020    18065      +45     
==========================================
+ Hits        16264    16334      +70     
+ Misses       1756     1731      -25     

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