Style fixes in fio_benchmark.py
Style fixes in fio_benchmark.py

Improve documentation and readability.

kivio/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,syed/PerfKitBenchmarker,syed/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,kivio/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker