Added pod affinity if the default ReadWriteOnce is used.

[keskival]

If accessMode ReadWriteOnce is used, then the volume can only be accessed in a write mode from a single node. This means that all pods which require write access to the volume need to be co-located to the same node. Failing that typically leads to a difficult to untangle locked volume mount situation where containers are stuck in ContainerCreating.

This change adds the necessary pod affinities if the related access modes are set in the values.yaml.