change the redundancy level of existing cluster and vdi

This is a new feature to allow user to change the redundancy level of
formatted cluster and created vdi. Only simple replica is supported now.

1. To set the replica number of formatted cluster:

    dog alter cluster-copy -c <copies>

2. To set the replica number of standalone vdi who has neither
   parent nor children:

    dog alter vdi-copy -c <copies> <vdiname>

3. To set the replica number of shared vdi who has parent or children,
   to avoid shared data inconsistency, please run dog vdi clone command
   with -n (--no-share) option first:

    dog vdi clone -s <snapshot> -n <src vdi> <dst vdi>

   It will deep copy a brand new standalone vdi base on source vdi
   so that we can run dog alter vdi-copy to change destinate vdi's
   replica number later.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>