Allow Zookeepers to be run with ephemeral storage.

[HoustonPutman]

Currently the operator only allows for persistent volumes, however for certain use cases it can be very useful to have ephemeral storage instead of persistent storage.

This could be implemented like the etcd-operator, where EmptyDir is used when no PVC spec is provided. The issue there is that the change would be backwards incompatible. The API documentation actually states that this is the behavior, but it isn't implemented.

The changes would be relatively minimal and would just require edits to the statefulSet generator and api.