Support journal logs in fluentd-gcp.

Only run the systemd-journal plugin when on a platform that requests it.
The plugin crashes the fluentd process if the journal isn't present, so
it can't just be run blindly in all configurations.

cluster/saltbase/salt/fluentd-gcp/fluentd-gcp.yaml

@@ -1,3 +1,5 @@
+# This config should be kept as similar as possible to the one at
+# cluster/addons/gci/fluentd-gcp.yaml
 apiVersion: v1
 kind: Pod
 metadata:
@@ -9,7 +11,7 @@ spec:
   dnsPolicy: Default
   containers:
   - name: fluentd-cloud-logging
-    image: gcr.io/google_containers/fluentd-gcp:1.20
+    image: gcr.io/google_containers/fluentd-gcp:1.21
     resources:
       limits:
         memory: 200Mi