[th/1000-vlans-test-cleanup] vlan: fix cleanup for @vlan_create_1000_bridges_over_1000_vlans test
A change in NetworkManager ([1]) will make it autoconnect more eagerly. Deleting the interfaces first (via iproute2), can lead to a new autoconnect of the vlan interfaces. Ensure we really get rid of those interfaces, by doing it twice.
This is a hack to get the test passing. It will be refactored later.