[stress testing] Investigate/fix/document object type mapping for matrix

There are cases where we may want to define, or at least document clear behavior, on how matrix yaml types translate to helm chart yaml types. For example, command args:

```
# matrix
matrix:
scenarios:
Test1:
command: bash
args:
- -c
- sleep 3600

# helm pod chart

template:
spec:
containers:
- image: busybox
command: {{ .Stress.command }}
args: {{ .Stress.args }} # array type
```

CC @ckairen @richardpark-msft