Helm: configure strategy for deployment

[joelluijmes]

Summary

Adds the possibility to the helm chart to configure the strategy of deployment.

Importance

Allows to configure rolling updates, for instance maybe someone (me) wants to tune the surge and unavailable parameters. Example of values.yaml

apollo:
  replicas: 3
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 0
      maxUnavailable: 1

  affinity: 
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchExpressions:
          - key: app.kubernetes.io/component
            operator: In
            values:
            - apollo
        topologyKey: kubernetes.io/hostname

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)

[zanieb]

Sweet thanks @joelluijmes ! I'm trying to decide if this is worth adding a section in the README but I feel like this is beyond the average user so it may not be worth it :)