admin hashes get regenerated on pod restart

The CouchDB Dockerfiles lay down an `[admins]` section in `/opt/couchdb/etc/local.d/docker.ini` [here](https://github.com/apache/couchdb-docker/blob/master/2.3.1/docker-entrypoint.sh#L68). The Helm chart is currently configured such that `/opt/couchdb/etc/default.d` is [persistent](https://github.com/apache/couchdb-helm/blob/e0d2ab9e8780a872c7537f88632893074bce5b92/couchdb/templates/statefulset.yaml#L92) but `/opt/couchdb/etc/local.d` is not.

This results in regeneration of the admin hashes whenever a CouchDB pod is restarted, invalidating any session cookies and leading to inconsistent auth failures with cookies are used.

I think the safest thing is likely to just make `/opt/couchdb/etc/local.d` persistent as well; the Dockerfile will already skip laying down a new `[admins]` section if one is present.

cc @kocolosk