build: Assert successful completion of cloud-init

cloud-init/user-data.template

@@ -18,7 +18,8 @@ runcmd:
   - truncate -s 0 /etc/hostname /etc/machine-id
   - rm -f /etc/ssh/ssh_host_* /var/lib/dbus/machine-id
   - ln -s /etc/machine-id /var/lib/dbus/machine-id
-  - poweroff
+  - echo "TEMPLATE_GENERATION_SUCCESS" | tee /var/log/template-generation-success
+  - sync
 
 disable_root: false
 users:
@@ -30,3 +31,8 @@ users:
       - ${PUBLIC_SSHKEY}
 ssh_authorized_keys:
   - ${PUBLIC_SSHKEY}
+
+power_state:
+  delay: now
+  mode: poweroff
+  condition: true