autopkgtest environments end up with duplicate default routes

[Steve Langasek]

With the migration to netplan/systemd-networkd/systemd-resolved, autopkgtest VMs now end up with multiple default routes. This causes miscellaneous autopkgtest failures of packages that assume a single record will be returned when grepping route -n output for 0.0.0.0.

These autopkgtests can be made more robust (it's not correct to assume that there will only be one default route on a system); but it's also a bug that there are duplicate routes.

This problem is reproducible in other artful VMs using netplan+networkd. E.g.:

$ gcloud compute instances create artful-test --image-project ubuntu-os-cloud-devel --image daily-ubuntu-1710-artful-v20170921a
Created [https://www.googleapis.com/compute/v1/projects/ubuntu-foundations/zones/us-west1-a/instances/artful-test].
NAME ZONE MACHINE_TYPE PREEMPTIBLE INTERNAL_IP EXTERNAL_IP STATUS
artful-test us-west1-a n1-standard-1 10.138.0.2 104.198.107.129 RUNNING
$ gcloud compute ssh artful-test
Warning: Permanently added 'compute.2478931335975611629' (ECDSA) to the list of known hosts.
Welcome to Ubuntu Artful Aardvark (development branch) (GNU/Linux 4.13.0-11-generic x86_64)

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 * Management: https://landscape.canonical.com
 * Support: https://ubuntu.com/advantage

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vorlon@artful-test:~$ ip route show
default via 10.138.0.1 dev ens4 proto dhcp src 10.138.0.2 metric 100
default via 10.138.0.1 dev ens4 proto dhcp metric 100
10.138.0.1 dev ens4 proto dhcp scope link src 10.138.0.2 metric 100
10.138.0.1 dev ens4 proto dhcp scope link metric 100
vorlon@artful-test:~$

[Dan Streetman (ddstreet)]

please reopen if this is still an issue