Update Elasticsearch custom resource to use Rolling Update Strategy

In the elasticsearch custom resource, the spec.updateStrategy is left blank, which defaults to OnDelete. When upgrading Istio and rolling the pods for Elasticsearch, you must manually delete the pods to have them recreated. By setting the update strategy to RollingUpdate, the pods will automatically cycle when the stateful set is told to restart.

The default should be RollingUpdate to facilitate upgrades to the Istio sidecar in Elasticsearch's pods.

Ref: https://www.elastic.co/guide/en/cloud-on-k8s/1.0/k8s-elasticsearch-k8s-elastic-co-v1.html

Ref: https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#pod-management-policies