provide restore hook

[hongchaodeng]

From https://sgotti.me/post/kubernetes-persistent-etcd/

So when the etcd-operator detects that a cluster has lost more than half of its nodes it cannot just wait for them to come back alive (since they don’t have persistent data and no stable peer address) but the only thing it can do is to drop the current cluster and create a new cluster restoring its data from a backup (if configured to do so). If this is enabled you’ll end up with an etcd cluster that under the hoods is restored with old data. So you’ve to be wise and know if this is a correct approach depending on what you’re storing inside your etcd kv store.

Auto-restore hides complexity and details. Sometimes users won't be comfortable with it. An initial idea is to provide restore hooks.

[hongchaodeng]

cc @sgotti

[hongchaodeng]

Closing this since we have a different design for restore. By default it needs human interaction