LUNetworkConnect/LUNetworkDisconnect, in case locking is used, might
lock instances that exist in the requested node group. The acquired
locks should be checked if they are correct at the beginning of
CheckPrereq() via _CheckNodeGroupInstances().

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>