Comment 3 for bug 2063331

Setting `optional: true` solves the boot delay.

```
$ sudo cat /etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource. Changes
# to it will not persist across an instance reboot. To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        end0:
            dhcp4: true
            optional: true
        end1:
            dhcp4: true
            optional: true
    version: 2
```

The boot is fast now:
```
$ systemd-analyze
Startup finished in 12.297s (kernel) + 8.067s (userspace) = 20.365s
graphical.target reached after 7.962s in userspace.
```