Explicitly set updateStrategy to RollingUpdate in statefulset

[hnicke]

PR Checklist

• [x] Chart Version bumped
• [x] Chart packaged and index updated (you can use ./release)
• [x] CHANGELOG.md updated

What this PR does / why we need it: This PR sets spec.updateStrategy.type of the StatefulSet to RollingUpdate.

Before the change:

$ kubectl get statefulset eventstore -o yaml

spec:
..
  updateStrategy:
    rollingUpdate:
      partition: 0
    type: RollingUpdate
..

$ kubectl rollout status statefulsets eventstore
Waiting for partitioned roll out to finish: 0 out of 1 new pods have been updated...

After the change:

$ kubectl get statefulset eventstore -o yaml

spec:
..
  updateStrategy:
    type: RollingUpdate
..

$ kubectl rollout status statefulsets eventstore
Waiting for statefulset spec update to be observed...
statefulset rolling update complete 1 pods at revision eventstore-7f77cd7c95...

Which issue this PR fixes *

There's a bug in kubernetes which we can circumvent if we specify RollingUpdate explictly (which is the default value).

Experienced this behaviour in kubernetes v1.11.3