Fix web options

[Trouv]

As mentioned in #1339, web_host and web_port aren't actually being used. Fixed this and added a test for it.

Still not sure about saying "*" is the default web_host when it no longer is. I changed the docstring for this anyway but idk if "" still means "all interfaces"?

[codecov]

Codecov Report

Merging #1340 into master will decrease coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   81.21%   81.06%   -0.16%     
==========================================
  Files          24       24              
  Lines        2231     2234       +3     
  Branches      338      339       +1     
==========================================
- Hits         1812     1811       -1     
- Misses        329      333       +4     
  Partials       90       90              

Impacted Files	Coverage Δ
locust/argument_parser.py	75.00% <ø> (ø)
locust/env.py	95.23% <ø> (ø)
locust/main.py	20.72% <0.00%> (-0.33%)	⬇️
locust/contrib/fasthttp.py	87.56% <0.00%> (-0.52%)	⬇️

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[heyman]

Looks good!

I ran into an issue that was caused by MacOS (Catalina) only setting up the loopback interface for 127.0.0.1. I suspect you're not running MacOS, and therefore I went ahead and made a work-around for it.

While at it, I also made a few other changes:

• The test now picks an available TCP port dynamically.
• I changed so that the default argument for --web-host is now '*' which will be replaced with '' in main.py. The reason for this is to be consistent with how --master-bind-host is specified.