MAAS needs to write power off jobs to to systemd units instead of upstart

just copying comment from the bug i opened.

$ grep -r etc.init etc/ contrib/
etc/maas/templates/commissioning-user-data/user_data_disk_erasing.template: cat >/etc/init/maas-poweroff.conf <<EOF
etc/maas/templates/commissioning-user-data/user_data_poweroff.template: cat >etc/init/maas-poweroff.conf <<EOF
etc/maas/templates/commissioning-user-data/user_data.template: cat >/etc/init/maas-poweroff.conf <<EOF
contrib/preseeds_v2/enlist_userdata: cat >/etc/init/maas-poweroff.conf <<EOF

Those are all places where maas writes a upstart job via cloud-init executed user-data.
Those will simply not execute in vivid now without upstart.

The result in all of those cases is that maas doesn't turn off or reboot.

In all cases, all maas really does is something like:
cat > /etc/init/maas-poweroff.conf <<EOF
   description "poweroff when maas task is done"
   start on stopped cloud-final
   console output
   task
   script
     [ ! -e /tmp/block-poweroff ] || exit 0
     poweroff
   end script
EOF

so maas will just need to write systemd jobs similarly. Its probably easiest and acceptable to just write both in all cases.