Split StatsD and Datadog config

[imiric]

This addresses the suggestion in this comment in order to close #671.

I tested the integration with a local StatsD instance and with Datadog and it works OK.

[codecov-commenter]

Codecov Report

Merging #1655 into master will decrease coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1655      +/-   ##
==========================================
- Coverage   72.16%   72.09%   -0.07%     
==========================================
  Files         166      165       -1     
  Lines       12787    12806      +19     
==========================================
+ Hits         9228     9233       +5     
- Misses       3009     3015       +6     
- Partials      550      558       +8     

Impacted Files	Coverage Δ
cmd/collectors.go	0.00% <0.00%> (ø)
stats/datadog/collector.go	74.19% <52.94%> (-25.81%)	⬇️
stats/statsd/collector.go	65.21% <55.55%> (-34.79%)	⬇️
cmd/config.go	82.35% <100.00%> (ø)
stats/statsd/common/collector.go	81.92% <100.00%> (ø)
stats/statsd/common/testutil/test_helper.go	97.75% <100.00%> (+0.02%)	⬆️
lib/executor/vu_handle.go	95.23% <0.00%> (+1.90%)	⬆️

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[mstoykov]

LGTM!
I was thinking of just having the parsing of the config to be in the corresponding package and that to be then transcoded to the common.Config and mostly no other changes. But I think I figured out there is a problem with that approach back when I left this unfinished and a little more code is not a problem :)