md: correct partial plug failure handling

If we plug multiple disks and some of them fail to purge directory, just call
md_del_disk and keep processing because successful plugging already add some
disks in the array.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: clean vdi_state when formatting cluster

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
zk: actively remove itself after leave_cluster() is called

actively remove itself from zookeeper than waitting for timeout.

Signed-off-by: ChenZhe <cz@de3eb.cn>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>