Add num_stress_threads as flags and metadata, benchmarks: cassandra
Add num_stress_threads as flags and metadata, benchmarks: cassandra

juju-solutions/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,lleszczu/PerfKitBenchmarker,gablg1/PerfKitBenchmarker,tvansteenburgh/PerfKitBenchmarker,emaeliena/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,syed/PerfKitBenchmarker,juju-solutions/PerfKitBenchmarker,lleszczu/PerfKitBenchmarker,ehankland/PerfKitBenchmarker,msurovcak/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,ksasi/PerfKitBenchmarker,kivio/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,emaeliena/PerfKitBenchmarker,tvansteenburgh/PerfKitBenchmarker,gablg1/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,ksasi/PerfKitBenchmarker,syed/PerfKitBenchmarker,askdaddy/PerfKitBenchmarker,ehankland/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,askdaddy/PerfKitBenchmarker,msurovcak/PerfKitBenchmarker,kivio/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker