dog/cluster: cluster-wide snapshot save or load specified vdi

It is useful for users who want to backup or restore only part of
all VDIs from cluster-wide snapshot.

To show which VDIs are snapshot, dog cluster snapshot show command
is added.

Usage and example:

1. save all vdi:
   dog cluster snapshot save snapname /tmp/ss

2. save some specified vdi:
   dog cluster snapshot save snapname /tmp/ss test0 test1 test2

3. load all vdi:
   dog cluster snapshot load snapname /tmp/ss

4. load some specified vdi:
   dog cluster snapshot load snapname /tmp/ss test6 test0 test3

5. show snapshot:
   dog cluster snapshot show snapname /tmp/ss

v4 is rebased on the latest master.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>