docs: def WaitForHostsReachable(hostnames, ssh_key):
    """ Blocks until host is reachable via ssh. """
    while True:
      unreachable = GetUnreachableHosts(hostnames, ssh_key)
      if unreachable:
        print 'waiting for unreachable hosts: %s' % unreachable
        time.sleep(5)
      else:
        break
    return