Support different Vault secret backend (e.g. database, ad)

It would be nice to be able to lookup secrets from the more robust secret engines in HashiCorp Vault, like the database and ad engines.  We allow HashiCorp Vault to rotate our database passwords and lazy rotation of Active Directory service account passwords.  I have a workaround to this, but it is not desirable because it adds complexity.

One possible implementation would be to allow for looking up a raw path, instead of parsing the path into <mount-point>/data/<remaining-path>.