dog/cluster: bug fix when cluster don't have snapshots

This patch fix the problem as below.

this is the first cluster snapshot I try to save, but I forgot to snapshot
the vdi:

dog cluster snapshot save first cluster_snapshots

Object not found. It may be caused by:
1. The cluster is empty.
2. All VDIs of the cluster don't have snapshots.

and that's right!

Then I take vdi snapshot and save cluster snapshot, then remove vdi snapshots.
Now I try to make the same mistake

dog cluster snapshot save second cluster_snapshots
100.0 % [=====] 4 / 4

There are no vdi snapshots and dog doesn't complain about it!
It did it only the first time.

Reported-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
test/func: add test case for cluster don't have snapshots

If all VDIs of the cluster don't have snapshots, dog should complain it
in any situation.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
cluster snapshot load with specified vdi, deleted vdi object not loaded

When we executing a cluster snapshot load command with specified vdi,
deleted vdi object should be also loaded for the reason of keeping
cluster consistancy.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
test/func: add test case for cluster snapshot load with specified vdi

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