RayElastic scale-up test fails

[tgaddair]

Followup from #2813, the ray elastic scale-up test is failing in Buildkite as well. We should investigate thus as part of #3190.

/usr/local/lib/python3.8/dist-packages/horovod/ray/elastic.py:454: RuntimeError
--
  | ------------------------------ Captured log call -------------------------------
  | ERROR    root:registration.py:179 failed to activate new hosts -> stop running
  | Traceback (most recent call last):
  | File "/usr/local/lib/python3.8/dist-packages/horovod/runner/elastic/registration.py", line 177, in _on_workers_recorded
  | self._driver.resume()
  | File "/usr/local/lib/python3.8/dist-packages/horovod/runner/elastic/driver.py", line 99, in resume
  | self._activate_workers(self._min_np)
  | File "/usr/local/lib/python3.8/dist-packages/horovod/runner/elastic/driver.py", line 177, in _activate_workers
  | pending_slots = self._update_host_assignments(current_hosts)
  | File "/usr/local/lib/python3.8/dist-packages/horovod/runner/elastic/driver.py", line 248, in _update_host_assignments
  | raise RuntimeError('No hosts from previous set remaining, unable to broadcast state.')
  | RuntimeError: No hosts from previous set remaining, unable to broadcast state.

https://buildkite.com/horovod/horovod/builds/6525#f16bee64-0b80-4cf8-9ba7-89b2d6aebde7/6-9500