Print valid URL when --web-host is not specified

[dmitrytokarev]

Currently if --web-host option is not specified the message is:
Starting web interface at http://:8089 which is an invalid URL and may confuse new users.

[codecov]

Codecov Report

Merging #1496 into master will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1496      +/-   ##
==========================================
+ Coverage   81.26%   81.32%   +0.05%     
==========================================
  Files          27       27              
  Lines        2386     2388       +2     
  Branches      366      367       +1     
==========================================
+ Hits         1939     1942       +3     
- Misses        353      355       +2     
+ Partials       94       91       -3     

Impacted Files	Coverage Δ
locust/main.py	18.66% <0.00%> (-0.19%)	⬇️
locust/runners.py	80.20% <0.00%> (-0.62%)	⬇️
locust/user/task.py	96.21% <0.00%> (+0.54%)	⬆️
locust/clients.py	95.09% <0.00%> (+4.90%)	⬆️

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[heyman]

Saying http://127.0.0.1:8089 might mislead people to believe that the server is only accepting connections from localhost.

In the case when web_host is */'', maybe it should say something like this instead:

Starting web interface at http://127.0.0.1:8089 (accepting connections from all network interfaces)

[dmitrytokarev]

@heyman I replaced 127.0.0.1 to 0.0.0.0 since . Added the note you proposed.

[heyman]

Nice!