Try to subscribe to a pool 3 times

Subscribing to a pool sometimes fails due to a race condition.  With
this change, the process happens 3 times before failing. And continue
normally as soon as the command works.
Looks pretty good. That said, you can bum out some of these lines of code, if you like.

```python
for _ in range(0, 3):
    result = run(...)
    if result.return_code is 0:
        return
    time.sleep(5)
print(...)
sys.exit(1)
```