Add a new function rollback volume to specified snapshot

Snapshot can provide a online backup and recover function.
Now we can make snapshot for a volume, this volume can be used to create new volume, It's just backup function.
Rollbak volume data to snapshot is the base functuion which cinder don't provide now, but it's useful in many cases.

New cinder api we be added, driver support is also important if we want to implement this function,
ceph has "rbd snap rollback" this interface in librbd,
LVM has "lvconvert --merge" can support this function.