Allow TopologyKey for anti-affinity rule to be changed

[345paul]

The topology key as set in this code (https://github.com/arangodb/kube-arangodb/blob/master/pkg/util/k8sutil/affinity.go) is hardcoded to use the label kubernetes.io/hostname.

It would be good to be able to have the option to set the value failure-domain.beta.kubernetes.io/zone so that the Arango pods are striped across availability zones as well as nodes, as described here: https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#inter-pod-affinity-and-anti-affinity-beta-feature