sheep: purge directory when plugging the new disk

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: abort when disk store is meta-store

This can stop sheep from rebooting when disk store is a meta-store, such as in a
disordered path parameter

$ sheep /disk1,/disk2,/meta-store # abort

We can only reboot when first component is meta-store to avoid mess-up.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>