Support snapshots encrypted with other master key.

To process encrypted snapshots of oher master key, one have to either add snapshot's master key or point to shapshot master key location. General idea is to bring snapshot keystore location to snapshot manager configuration. These master keys will never be used to encrypt persistent db, but will be involved to decrypt snapshots. Snapshot master key can be recognized by name/digest. If not registered, dedicated snapshot keystore is searched for the master key.