Blocks until host is reachable via ssh.

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