Ensure all images can be modified with environment variables

[lucasmdrs]

Use envconfig to set defaults and ensure all images used can be defined with environment variables

[codecov-commenter]

Codecov Report

Merging #272 (0cc92f8) into master (d18cdd3) will decrease coverage by 0.07%.
Report is 1 commits behind head on master.
The diff coverage is 33.33%.

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@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
- Coverage   56.16%   56.10%   -0.07%     
==========================================
  Files          37       37              
  Lines        4136     4112      -24     
==========================================
- Hits         2323     2307      -16     
+ Misses       1690     1682       -8     
  Partials      123      123              

Files Changed	Coverage Δ
pkg/backends/ghz/backend.go	39.84% <ø> (+1.20%)	⬆️
pkg/backends/jmeter/backend.go	39.06% <ø> (-3.23%)	⬇️
pkg/backends/k6/backend.go	38.29% <ø> (+1.05%)	⬆️
pkg/backends/locust/backend.go	67.53% <0.00%> (-0.83%)	⬇️
pkg/backends/jmeter/resources.go	69.15% <50.00%> (ø)

[fabioserra]

Please update the documentation for the env variables for locust
https://github.com/hellofresh/kangal/blob/master/docs/env-vars.md?plain=1#L57