System keyspaces improvements

Load travel sample
Create user marco with query select on travel-sample.itinerary.airlines

both

select * from system:buckets

and

select * from system:scopes

return no documents.
Technically this is correct as marco doesn't have privileges on travel-sample or itinerary, but it would probably be more informative if we returned `travel-sample` and `travel-sample`.itinerary respectively given that we can access at least one collection in there.

All the keyspace keyspaces (indexes, keyspaces, buckets, scopes) could use GetUserBuckets() rather than namespace.Objects(), as it would cut down on scanning buckets the user doesn't have access to.

system:nodes should report the node uuid.