Change LUQueryNodes to return raw values and support selective listing

LUQueryNodes it's very similar to LUQueryNodeData, but it lacks two
features:
  - instance list (it has count though), both primary and secondary
  - selective node listing

In order to support these features, we change it to return raw values
instead of stringified ones (like the recent change to LUQueryInstances)
and to support query-ing of a restricted set of nodes.

This CL also modifies the gnt-node script to conform to the new protocol
and the opcode OpQueryNodes to support the new "nodes" attribute.

Reviewed-by: imsnah