fixes invalid null selector issue

Before this change this example would result in: 

$ kubectl create -f sysdig.yml
The DaemonSet "sysdig-agent" is invalid.
spec.template.metadata.labels: Invalid value: null: `selector` does not match template `labels`

examples/sysdig-cloud/sysdig-daemonset.yaml

@@ -8,6 +8,9 @@ metadata:
     app: sysdig-agent
 spec:
   template:
+    metadata:
+      labels:
+        name: sysdig-agent
     spec:
       volumes:
       - name: docker-sock