Fix locking in LUNetworkConnect()

Locks for group instances are aquired only if conflicts are checked.
To this end we must _CheckNodeGroupInstances() only then otherwise
this check will always fail (owned_instances will be []).

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>