Fine tunning siege benchmark options

[racardoso]

Summary:

To set siege parameters for benchmark, oss-performance uses static
functions defined on PerfSettings.
It's seems more easy if we can do this via command line rather than
edit file.

This commit introduces three new command line options:

--benckmark-time
--benchmark-concurrency
--benchmark-warm-concurrency

Those functions permits to set benchmark time, benchmark concurrency
and, warmup (multi) concurrency values. Oss-performance will use default
values if you don't pass anything or pass a invalid value.

Those options doen't affect single request warmup stage.

[fredemmott]

Sorry for the delay: this should be modified so that --i-am-not-benchmarking is specified. Even if slightly inaccurate, we've had huge problems with people running with different settings without telling anyone. Anything that significantly affects performance beyond the defaults should be covered by that option.

[racardoso]

Ok i will made those changes and submit.