parser: redefining VLAN interface away from OVS parent results in wrong backend

[Simon Chopin]

Consider the following netplan configuration:

# /etc/netplan/a.yml
network:
  ethernets:
    eth0:
      openvswitch:
        ports:
          - [patch0-1, patch1-0]
    eth1:
      dhcp4: true
  vlans:
    vlan15:
      id: 15
      link: eth0

# /etc/netplan/b.yml
network:
  vlans:
    vlan15:
      link: eth1

The expected result would be vlan15 handled by the default renderer, however with the current code it will be handled by OVS.

I could see two acceptable behaviors for this:
* actually do what the user should expect and use the proper renderer, totally discarding the link: eth0 line from a.yml
* error out when trying to redefine a field that's been defined already

I think we already have instances of the latter behavior for other fields? I find it a relatively sensible way to handle this :)

[Lukas Märdian (slyon)]

Another idea would be to set the (implicit) OVS renderer in a "postprocessing" stage, after processing the explicit YAML settings, but before validation. And do it only once after all the explicit data has been parsed.