Comment 12 for bug 1929560

This backport to 18.04 LTS broke all our OpenStack compute nodes. We use systemd-network to configure interfaces etc. and neutron-linuxbridge-agent with some flat networks. In this scenario, the neutron agent creates bridges for the flat networks and assigns them to physical interfaces based on a mapping.

The backport now sets nomaster on these interfaces, effectively removing the bridges and cutting of connectivity of all VMs using these flat networks. This happened already when the systemd package was upgraded (and systemd-networkd restarted).

Example:

```
10: flat-net1@data: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master brq88363244-5f state UP group default qlen 1000
    link/ether b0:22:28:1c:f7:12 brd ff:ff:ff:ff:ff:ff
```

The interface (VLAN here) is created and managed by systemd-networkd, the bridge `brq88363244-5f` is managed by neutron. Of course, systemd-networkd doesn't know anything about the bridge, this wasn't a problem until now, but the latest patch update always sets nomaster to `flat-net1`. We were really surprise by this change in 18.04 LTS.

Is there any recommend workaround to have systemd-networkd not set nomaster or do we have to not use systemd-network at all?