Make possible for the cloud output to stop the engine

[mstoykov]

This does add an interface that can be implemented by other outputs as well.

fixes #1880

TODOs:

• Figure out a good name. I kept changing what I think this should be called through the writing of this and am not certain I like any of the names much ... I would argue that the best one is StopExecutionOnOutputSignal as it makes it more accurate that we will over stop if the output couldn't be started, and that this is a signal from the output to stop not just that there was some error. Unfortunately, it's a bit of a mouthful
• Figure out how to write an integration test of this ... I have tested it manually and it works (and takes 3 minutes to test) but it will be nice to have a full integration test about this.
• Print which output signaled to close

Related to the last point, I decided to have an error be "returned" not just to call some function is so that we can always print it in some good known format instead of it being a responsibility of the output.

[na--]

I'll get back to you on the names later, but please make this a cloud output option, not a runtime option.

[codecov-io]

Codecov Report

Merging #1965 (f59b2ee) into master (311b905) will decrease coverage by 0.05%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #1965      +/-   ##
==========================================
- Coverage   71.48%   71.43%   -0.06%     
==========================================
  Files         183      183              
  Lines       14247    14257      +10     
==========================================
  Hits        10184    10184              
- Misses       3432     3442      +10     
  Partials      631      631              

Flag	Coverage Δ
ubuntu	71.35% <40.00%> (-0.05%)	⬇️
windows	71.04% <40.00%> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
core/engine.go	83.25% <0.00%> (-1.88%)	⬇️
output/cloud/output.go	81.15% <66.66%> (-0.13%)	⬇️
cloudapi/config.go	85.54% <100.00%> (+0.35%)	⬆️
lib/executor/ramping_vus.go	93.80% <0.00%> (-1.24%)	⬇️
lib/executor/vu_handle.go	94.28% <0.00%> (-0.96%)	⬇️

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[imiric]

LGTM, just some minor comments.

Re: the name, StopRunWithError sounds better to me than StopExecutionOnOutputSignal, as "signal" has the Unix signal connotation. Though I would add Engine to it since that's what happens, so StopEngineRunWithError?

And I agree with the need for an integration test, but it could be done in a follow up PR.