Comment 2 for bug 1709135

Sounds like we'd need to add a key under parameters: for the bond device, as per the original description (but the device needs to be listed in 'interfaces', and exist in the list of interfaces defined in netplan:

ethernets:
  version: 2
  ethernets:
    eth0:
      match:
        driver: virtio
    ens4:
      match:
        driver: e1000
  bonds:
    bond0:
      interfaces: [ eth0, ens4 ]
      parameters:
        mode: active-backup
        mii-monitor-interval: 100
        primary: ens4
      dhcp4: true

This will require backporting the feature from systemd to the releases where we need this (to xenial).