Support delegation to external load testing tools

[gmemcc]

hi @stefanprodan , we found it helpful to add support for external load testing tools. in this PR loadtester package was refactored, a new test task type ngrinder was added to suport the popular performance testing platform nGrinder
please take a look, thanks:D

[stefanprodan]

Hi @gmemcc this looks great. Can you please add a section in here and explain how this works with nGrinder.

Thanks

[codecov-io]

Codecov Report

Merging #90 into master will increase coverage by 1.66%.
The diff coverage is 57.03%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage    54.8%   56.46%   +1.66%     
==========================================
  Files          13       16       +3     
  Lines        1540     1801     +261     
==========================================
+ Hits          844     1017     +173     
- Misses        556      626      +70     
- Partials      140      158      +18

Impacted Files	Coverage Δ
pkg/loadtester/server.go	0% <0%> (ø)	⬆️
pkg/loadtester/task.go	100% <100%> (ø)
pkg/loadtester/runner.go	91.42% <100%> (+8.44%)	⬆️
pkg/loadtester/task_ngrinder.go	55.17% <55.17%> (ø)
pkg/loadtester/task_shell.go	76.19% <76.19%> (ø)
pkg/controller/deployer.go	57.92% <0%> (-0.79%)	⬇️
pkg/controller/job.go	0% <0%> (ø)	⬆️
... and 2 more

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[gmemcc]

Hi @stefanprodan, I added a paragraph at the end of how-it-works.md, please take a look, thanks.

[stefanprodan]

LGTM

Thanks @gmemcc