Arbitrary JSON objects in generator_params

[jelu]

          I would go for an arbitrary JSON object. It's up to generator how it encodes it's parameters.

Originally posted by @pspacek in #2 (comment)

[pspacek]

I did not realize the schema by default allows arbitrary properties. I think that mostly covers it.

With this in mind I would define an optional 'cmdline' property in the header 'header' object, make it an array of strings, and leave it up to generators to add whatever other properties they want or need.

cmdline should be optional because sometimes the command line does not exist - e.g. when the workload is generated by a library. Think e.g. of dnsjit used to send stuff to the server.