g5k: Fix Kavlan error handling when re-applying the same VLAN config

I missed a case in the previous commit: when the node is already in the right VLAN, the G5K API returns an "unchanged" status such as the following:

{'ecotype-43-eth1.nantes.grid5000.fr': {'status': 'unchanged',
 'message': 'Node already in vlan'}}

This case was causing an exception to be thrown, while there's really no reason to do that.