LUNetworkConnect/Disconnect looks up a nodegroup's instances for
conflicting IPs. To do so, use GetNodeGroupInstances() and
GetMulitInstanceInfo().

Additionally, check if the correct locks were acquired.

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