allow unmanaged NICs

[bmwiedemann]

Why is this change necessary? We want to allow network interfaces that are not managed by chef

How does it address the issue? we keep interfaces around when the config file has an "#unmanaged" marker comment

Is there additional information worth sharing like links to a Trello card, bug references, testing advice or dependencies to other pull requests? vuntz said, there is a FATE or bug for this and also Bernhard needs this for running SOC on multicast-vxlan overlay networks where the underlying NIC (e.g. eth0) still needs to work.

This also includes a cleanup commit that is useful without the 2nd one

[vuntz]

I'm nearly sure we wanted to do that in network.json...