config: write vlan network interface

config: write vlan network interface

* Add vlan_id and vlan_raw_device fields to the Interface api type
* Write to the network config the vlan specific properties for vlan
  interface type
* Add several tests to verify the functionally

Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
Tested-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>
Tested-by: Folke Gleumes <f.gleumes@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>