Do not fail if we have vlan interface with unexpected naming

When trying to create eth0.248, if the interface already exists but as
vlan248, we simply fail. It turns out there's code already that is
supposed to deal with that and remove vlan248, but it's executed too
late.
Use kill_nic to destroy the other vlan interface

We want to remove the configuration files too.
Merge pull request #199 from vuntz/force-vlan-ifname

Do not fail if we have vlan interface with unexpected naming