Shutdown tests unintentionally sleep for 10 seconds

Some of the tests in `test/functional/master/test_shutdown.py` do this at the end of the test:
```python
self.cluster.block_until_n_slaves_dead(2, 10)
```

The `block_until_n_slaves_dead` method does this:
```python
def block_until_n_slaves_dead(self, num_slaves, timeout):
...
def are_slaves_dead():
are_n_slaves_dead(num_slaves)

slaves_died_within_timeout = poll.wait_for(are_slaves_dead, timeout_seconds=timeout)
...
```

The problem is that that `are_slaves_dead()` function doesn't return anything (so it always returns None). That makes the `poll.wait_for` call always take the full 10 seconds. (There is a separate issue in that the return value from the `self.cluster.block_until_n_slaves_dead(2, 10)` call is not asserted on which would have also caught this issue.)

From the git history it looks like these calls were added to address a flaky test in #174, so that indicates that some amount of sleep is necessary, but not the whole 10 seconds.