Allow locust to get SIGTERM(aka Ctrl+C) messages.

[turgayozgur]

exec used to start so from now, it will be the top process and can handle the SIGTERM messages.

[codecov]

Codecov Report

Merging #1159 into master will decrease coverage by 0.89%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1159     +/-   ##
=========================================
- Coverage   76.23%   75.33%   -0.9%     
=========================================
  Files          20       20             
  Lines        1889     1889             
  Branches      293      293             
=========================================
- Hits         1440     1423     -17     
- Misses        379      394     +15     
- Partials       70       72      +2

Impacted Files	Coverage Δ
locust/util/deprecation.py	75% <0%> (-10%)	⬇️
locust/contrib/fasthttp.py	87.42% <0%> (-2.29%)	⬇️
locust/runners.py	63.18% <0%> (-1.65%)	⬇️
locust/web.py	86.82% <0%> (-1.56%)	⬇️
locust/main.py	33.47% <0%> (-1.31%)	⬇️

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[heyman]

Thanks!