Comment 6 for bug 1449318

I'm able to work around this on 15.10 by applying smoser's fix as part of my cloud config:

#cloud-config
power_state:
  mode: poweroff
runcmd:
  - set -ex
  - systemctl daemon-reload
  - python3 -c "raise Exception"
write_files:
  - path: /etc/systemd/system/cloud-final.service.d/override.conf
    content: |
      [Service]
      # See https://bugs.launchpad.net/cloud-init/+bug/1449318
      KillMode=process