blockstore loadtests: extracted common TStorageServiceConfig initialization code to a function, limiting bw by 32MiB/s in this function, using it in network-ssd/network-hdd loadtests

blockstore loadtests: extracted common TStorageServiceConfig initialization code to a function, limiting bw by 32MiB/s in this function, using it in network-ssd/network-hdd loadtests #1454