Enable `per_repository` elector in Praefect

Praefect's has recently gained support for repository specific primaries as opposed to electing a primary for the whole virtual storage. This allows Praefect better spread write load across the cluster, pick the best primary for each repository. It's also a requirement for features such as variable replication factor as not every storage will contain every repository anymore. This commit enables the elector in the QA tests prior to enabling it in production.