builder cloudconfig: shutdown server after 3 hours (#1554)

buildman/templates/cloudconfig.yaml

@@ -77,3 +77,12 @@ coreos:
         Where=/var/lib/docker
         Type=btrfs
         Options=loop,discard
+    - name: machine-lifetime.service
+      command: start
+      enable: yes
+      content: |
+        [Unit]
+        Description=Machine Lifetime Service
+        [Service]
+        Type=oneshot
+        ExecStart=/bin/sh -xc "/bin/sleep 10800; /usr/bin/systemctl --no-block poweroff"