ability to specify secret and data name within secret

[lknite]

When initially deploying I used cloudnative-pg to deploy a postgres database.

After the postgres database is deployed there is a secret like so:

$ k get secrets mattermost-cluster-app -o yaml
apiVersion: v1
data:
  dbname: redacted_base64
  fqdn-jdbc-uri: redacted_base64
  fqdn-uri: redacted_base64
  host: redacted_base64
  jdbc-uri: redacted_base64
  password: redacted_base64
  pgpass: redacted_base64
  port: redacted_base64
  uri: redacted_base64
  user: redacted_base64
  username: redacted_base64

Via the mattermost strategy, I can specify a secret ... so I could specify the secret provided by cloudnative-pg, but I would need the ability to say to use the 'uri' value for the DB_CONNECTION_STRING value.

This would allow for a fully automated deployment, rather than to have to copy/paste the secret value from the cloudnative-pg generated secret, to the mattermost secret.