api: storage create/update: return parts of the configuration

api: storage create/update: return parts of the configuration

First, doing such things can make client work slightly easier, as the
submitted values do not need to be made available in any callback
handling the response.

But the actual reason for doing this now is, that this is a
preparatory step for allowing the user to download/print/.. an
autogenerated PBS client encryption key.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>