$ ud=$(printf "#cloud-config\npackage_upgrade: true\n")
$ lxc image show ubuntu-daily:4c7d721fe970
autoupdate: false
properties:
  aliases: 16.10,devel,y,yakkety
  architecture: amd64
  description: ubuntu 16.10 amd64 (daily) (20160902)
  label: daily
  os: ubuntu
  release: yakkety
  serial: "20160902"
  version: "16.10"
public: true

$ lxc delete --force y1 &>/dev/null
$ lxc launch ubuntu-daily:4c7d721fe970 "--config=user.user-data=$ud" y1
Creating y1
Starting y1

$ lxc exec y1 -- tail -f /var/log/cloud-init-output.log