Setting existingSecret for PostgreSQL is not respected by our default values

[tumido]

Describe the bug

Setting upstream.postgresql.auth.existingSecret is respected by the PostgreSQL subchart, however we override it for POSTGRESQL_ADMIN_PASSWORD in our default values. We do not respect the existingSecret value here. We also do not respect the postgresPassword for that matter.

Note: POSTGRESQL_ADMIN_PASSWORD is not set by the bitnami/postgres chart, however this is a required env variable for UBI-based Postgres images.

Expected Behavior

What are the steps to reproduce this bug?

1. helm template --set upstream.backstage.postgresql.auth.existingSecret=secretName janus-idp/backstage
2. Look at the environment variables exposed to the PostgreSQL container:

...
# Source: backstage/charts/upstream/charts/postgresql/templates/primary/statefulset.yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: release-name-postgresql
  ...
spec:
  ...
  template:
    ...
    spec:
      ...
      containers:
        - name: postgresql
          ...
          env:
            ...
            # Authentication
            - name: POSTGRES_USER
              value: "bn_backstage"
            - name: POSTGRES_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: secretName
                  key: password
            - name: POSTGRES_POSTGRES_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: secretName
                  key: postgres-password
            - name: POSTGRESQL_ADMIN_PASSWORD
              valueFrom:
                secretKeyRef:
                  key: postgres-password
                  name: 'release-name-postgresql'

Versions of software used and environment

[tumido]

Workaround is to use values:

upstream:
  backstage:
    extraEnvVars:
      - name: POSTGRESQL_ADMIN_PASSWORD
        valueFrom:
          secretKeyRef:
            key: postgres-password
            name: <EXISTING_SECRET>
  postgresql:
    primary:
      extraEnvVars:
        - name: POSTGRESQL_ADMIN_PASSWORD
          valueFrom:
            secretKeyRef:
              key: postgres-password
              name: <EXISTING_SECRET>