EC2: integrate KeyPairs responses with core serializer

[bpandola]

• model typing and attribute name fix-ups
• test assertions added for better coverage
• conditional response field helper methods moved from ECS backend to BaseResponse for use with IncludePublicKey parameter
• XML templates deleted

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.03%. Comparing base (687142f) to head (2a0f060).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9465   +/-   ##
=======================================
  Coverage   93.02%   93.03%           
=======================================
  Files        1293     1293           
  Lines      116237   116252   +15     
=======================================
+ Hits       108134   108151   +17     
+ Misses       8103     8101    -2     

Flag	Coverage Δ
servertests	28.82% <33.33%> (+<0.01%)	⬆️
unittests	93.00% <100.00%> (+<0.01%)	⬆️

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[bblommers]

Thanks for the cleanup @bpandola! ❤️