text: Title: Key Error: 'ib0'
  Body: **Environment:**
  1. Framework: pytorch
  2. Framework version:1.8.0
  3. Horovod version:0.21.3
  4. MPI version: mpich 3.0.4
  5. CUDA version: 10.1
  6. NCCL version:
  7. Python version:
  8. Spark / PySpark version: 
  9. Ray version:
  10. OS and version:
  11. GCC version:
  12. CMake version:
  
  when I run `horovodrun -np 1 --start-timeout=180 --min-np 1 --max-np 3 --host-discovery-script ./discover_hosts.sh python -u pytorch_mnist_elastic.py `, I got the following error
  
  
  **Bug report:**
  Traceback (most recent call last):
    File "/home/test/dat01/txacs/anaconda3/envs/py36/bin/horovodrun", line 8, in <module>
      sys.exit(run_commandline())
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/launch.py", line 768, in run_commandline
      _run(args)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/launch.py", line 756, in _run
      return _run_elastic(args)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/launch.py", line 666, in _run_elastic
      gloo_run_elastic(settings, env, args.command)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/gloo_run.py", line 336, in gloo_run_elastic
      launch_gloo_elastic(command, exec_command, settings, env, get_common_interfaces, rendezvous)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/gloo_run.py", line 303, in launch_gloo_elastic
      server_ip = network.get_driver_ip(nics)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/util/network.py", line 100, in get_driver_ip
      for addr in net_if_addrs()[iface]:
  KeyError: 'ib0'
  Launching horovod task function was not successful:
  
  Exception in thread Thread-10:
  Traceback (most recent call last):
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/threading.py", line 916, in _bootstrap_inner
      self.run()
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/threading.py", line 864, in run
      self._target(*self._args, **self._kwargs)
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/util/threads.py", line 58, in fn_execute
      res = fn(*arg[:-1])
    File "/home/test/dat01/txacs/anaconda3/envs/py36/lib/python3.6/site-packages/horovod/runner/driver/driver_service.py", line 87, in _exec_command
      os._exit(exit_code)
  TypeError: an integer is required (got type NoneType)
  
  Launching horovod task function was not successful:
  
  
  
  Please help me, thanks!
  
  