collectd: Start after install

The *collectd.service* unit is now starged automatically after it is
installed on first boot.

collectd.yaml

@@ -4,7 +4,7 @@ version: 1.4.0
 ignition:
   config:
     merge:
-    - local: packages.yaml
+    - local: packages.ign
 
 storage:
   files:
@@ -73,6 +73,10 @@ storage:
         [Install]
         WantedBy=multi-user.target
 
+  links:
+  - path: /etc/systemd/system/after-install.target.wants/collectd.service
+    target: /usr/lib/systemd/system/collectd.service
+
 systemd:
   units:
   - name: collectd.service