shepherd: show warning at startup

The current shepherd driver doesn't handle node change events as sheep
expects.  This patch prints a warning message to notice the user that
shepherd is not suitable for production, and adds some FIXME comments
that must be done before we remove the warning message.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>