Comment 2 for bug 1804701

On a machine with LXDs as well, the upgrade prepare completed OK but the complete didn't, with this in the machine log:

2018-11-23 03:28:43 INFO juju.packaging.manager utils.go:105 Retrying: apt-get --option=Dpkg::Options::=--force-confold --option=Dpkg::options::=--force-unsafe-io --assume-yes --quiet install --target-release trusty-backports lxd
2018-11-23 03:28:53 INFO juju.packaging.manager utils.go:105 Retrying: apt-get --option=Dpkg::Options::=--force-confold --option=Dpkg::options::=--force-unsafe-io --assume-yes --quiet install --target-release trusty-backports lxd
2018-11-23 03:28:53 ERROR juju.packaging.manager utils.go:109 packaging command failed: exit status 100; cmd: "apt-get --option=Dpkg::Options::=--force-confold --option=Dpkg::options::=--force-unsafe-io --assume-yes --quiet install --target-release trusty-backports lxd"; output: Reading package lists...
E: The value 'trusty-backports' is invalid for APT::Default-Release as such a release is not available in the sources

2018-11-23 03:28:53 WARNING juju.provisioner container_initialisation.go:145 not stopping machine agent container watcher due to error: setting up container dependencies on host machine: packaging command failed: exit status 100
2018-11-23 03:28:53 ERROR juju.provisioner container_initialisation.go:124 starting container provisioner for lxd: setting up container dependencies on host machine: packaging command failed: exit status 100
2018-11-23 03:28:56 INFO juju.provisioner container_initialisation.go:116 initial container setup with ids: [0/lxd/14 0/lxd/2 0/lxd/4 0/lxd/1 0/lxd/10 0/lxd/11 0/lxd/5 0/lxd/6 0/lxd/7 0/lxd/9 0/lxd/0 0/lxd/12 0/lxd/3]