(QENG-807) add --collect-perf-data option, rspec tests, and yard docs

If beaker is run with "--collect-perf-data" it will ensure sysstat is installed and configured on the Linux hosts before testing begins. After testing is finished, a sar report is sent to logging for each Linux host.
I don't think that @collect_perf_data boolean should be here - you should only initialize the Perf object if that variable is True, you shouldn't be checking it at all from within the object.