Currently, OpQueryNodes is locking all nodes (in shared mode), which
will also block the special case of querying only for the node names
(this is needed for gnt-cluster command, for example). There is no
logical requirement to not give the administrator enough power if she/he
knows what to do, so it would be logical that querying the node names
works without a lock.

The patch changes the LUQuerytNodes.ExpandNodes to only request locking
when the selected fields contain dynamic attributes, and a small change
in the Exec() method to use the new, atomic query method from
ConfigWriter.

Reviewed-by: ultrotter