Apply filter properly in LUQuery{Nodes, Instances}

Currently when not locking all nodes/instances are returned, regardless
if the user asked only for some of them. With this patch we return to
the previous behaviour:
  - if no names are specified return info on all current ones
  - if some names are specified and are not found give an error
  - otherwise return only info on the specified names

Reviewed-by: iustinp