Returns list of hosts unreachable via ssh.

def GetUnreachableHosts(hostnames, ssh_key):
  """ Returns list of hosts unreachable via ssh. """
  ssh_status = AreHostsReachable(hostnames, ssh_key)
  assert(len(hostnames) == len(ssh_status))
  nonresponsive_hostnames = [host for (host, ssh_ok) in
                             zip(hostnames, ssh_status) if not ssh_ok]
  return nonresponsive_hostnames