Comment 10 for bug 1899487

Comment #8 made me think about why this works in our functional test, so I went to investigate that. In our functional test we use Bionic images, and sure enough the netplan.yaml [9] written there does _NOT_ include the MTU! The OpenStack Metadata source remains the same [10].

9: ubuntu@banana-1:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.5 LTS
Release: 18.04
Codename: bionic
ubuntu@banana-1:~$ 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:
        ens2:
            dhcp4: true
            match:
                macaddress: fa:16:3e:a3:34:78
            set-name: ens2
    version: 2

10: $ curl http://169.254.169.254/openstack/2018-08-27/network_data.json
{"links": [{"id": "tapc352887e-0f", "vif_id": "c352887e-0fff-481b-af47-7df9f7c2ff05", "type": "ovs", "mtu": 8950, "ethernet_mac_address": "fa:16:3e:a3:34:78"}], "networks": [{"id": "network0", "type": "ipv4_dhcp", "link": "tapc352887e-0f", "network_id": "f8123ceb-e29d-4f4a-b200-6fb3bf3984ba"}], "services": []}