sheep: fix bug for not saving block_size_shift to cluster config

This patch fixes bugs that block_size_shift info was forgotten
after cluster shutdown and start sheepdog.

Add block_size_shift info to cluster config file.

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>