Issues with yaml list parsing

```
19:39:47 - INFO - cbt - Running rbd fio baseline_latency test, mode ['randwrite', 'randread']
19:39:47 - ERROR - cbt - During tests
Traceback (most recent call last):
File "/home/ubuntu/cbt/cbt.py", line 92, in main
b.run()
File "/home/ubuntu/cbt/benchmark/librbdfio.py", line 245, in run
self.run_workloads()
File "/home/ubuntu/cbt/benchmark/librbdfio.py", line 186, in run_workloads
self.run_dir = ( f'{self.base_run_dir}/{self.mode}_{int(self.op_size)}/'
^^^^^^^^^^^^^^^^^
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'list'
```

The test in question is
```
baseline_latency:
jobname: "1_latency_floor"
mode: ["randwrite", "randread"] # Test both R and W
op_size: [4096, 8192] # 4K (Standard), 8K (Postgres Page)
iodepth: [1] # STRICTLY 1
numjobs: [1] # STRICTLY 1 process per client
procs_per_client: [1]
```