fix #3014: allow DataStoreAdmins to list DS config

fix #3014: allow DataStoreAdmins to list DS config

filtered by those they are privileged enough to read individually. this
allows such users to configure prune/GC schedules via the GUI (the API
already allowed it previously).

permission-wise, a user with this privilege can already:
- list all stores they have access to (returns just name/comment)
- read the config of each store they have access to individually
(returns full config of that datastore + digest of whole config)

but combines them to
- read configs of all datastores they have access to (returns full
config of those datastores + digest of whole config)

user that have AUDIT on just /datastore without propagate can now no
longer read all configurations (but this could be added it back, it just
seems to make little sense to me).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>