sysinfo.ecpp should use cxxtools to build up JSON output

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see PR #488

@geraldguillaume and @barraudl please add references if possible, thx

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Comment from @geraldguillaume
It would make sense to rewrite the JSON output payload by using helper serializer method provided by cxxtools::JsonSerializer
http://www.tntnet.org/howto/json-howto.html