This PR adds DeletionPolicy which describes a policy for end-of-life maintenance of volume snapshot contents.
There are two types of policies, delete and retain. By default, the
policy is delete. In this case, when volume snaphsot is deleted, the
volume snapshot content for it should be deleted, as well as the
physical snapshot. If the policy is set the retain, the volume snapshot
content will remain even if the volume snapshot it binds to is deleted.
