prscript: rework option logic

prscript: rework option logic

Handle cleanly the case where user start docker handling command
without having the dependency installed. The help message does
now say:

```
  -d, --docker          use docker based testing
  -C, --create          create docker container (disabled)
  -s, --start           start docker container (disabled)
  -S, --stop            stop docker container (disabled)

You need to install Python docker module to enable docker container handling
options.
```

And running a disabled options says:

```
$ qa/prscript.py -s
You need to install python docker to use docker handling features.
```