vault design doc updates

@craig @ahanselka I added some clarifications around non-goals, use cases, unsealing and integration into the product.

One thing that still needs clarification is the storage backend. I think the reason that lead to deciding against Consul isn't valid (https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6890#note_180760324). So we should re-evaluate Consul as storage backend again as it is also the reference architecture for Vault.