EC2: create_fleet() now correctly handles Overrides with a single value

[bblommers]

Fixes #8311

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (c9536b1) to head (a654323).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8316   +/-   ##
=======================================
  Coverage   94.56%   94.57%           
=======================================
  Files        1159     1159           
  Lines      100881   100886    +5     
=======================================
+ Hits        95403    95408    +5     
  Misses       5478     5478           

Flag	Coverage Δ
servertests	28.86% <0.00%> (-0.01%)	⬇️
unittests	94.54% <100.00%> (+<0.01%)	⬆️

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