Add RollingUpdate strategy to dockergc deployment config

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1599341

roles/openshift_docker_gc/templates/dockergc-ds.yaml.j2

@@ -14,6 +14,8 @@ items:
     labels:
       app: dockergc
   spec:
+    updateStrategy:
+      type: RollingUpdate
     template:
       metadata:
         labels: