VLAN interfaces should have device autodetected

VLAN interfaces can be created by stating the parent device in the VLAN definition, but the device should be applied automatically if the VLAN is a member of an interface.

For instance, this works:

-
  type: interface
  name: nic2
  addresses:
    - ip_netmask: 192.168.20.10/24
-
  type: vlan
  vlan_id: 20
  device: nic2

But the device: nic2 shouldn't be required. It would be better if type interface accepted a members: stanza, and the VLAN could be a member. The device could be determined by the parent object (interface in this case).