Set locust parameters via env vars & config file

[cyberw]

It would be useful to be able to set locust's command line parameters via env vars (particularly to be able to provide defaults, to shorten the command that you have to type).

I suggest we use LOCUST_ for env vars.

Command line should override env vars of course.

I can make a PR if this sounds good to everyone. I'd probably use https://pypi.org/project/ConfigArgParse/ to do it.

[heyman]

I can't think of any drawbacks so I'm +1/+0.

[cyberw]

PR:ed. What does +0 mean? :)

[heyman]

What does +0 mean? :)

I believe it's the Django project that uses it (or at least used to use it) to indicate that you are positive to something, but don't feel strongly about it (it wouldn't matter so much to you that you're willing to champion it). I think they have the policy that a -0 (or -1 obviously) from any core developer would have to be converted to a +0 or +1 before it can get implemented.

[cyberw]

Fixed!