Shutdown tests unintentionally sleep for 10 seconds

[josephharrington]

Some of the tests in test/functional/master/test_shutdown.py do this at the end of the test:

self.cluster.block_until_n_slaves_dead(2, 10)

The block_until_n_slaves_dead method does this:

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.