Helm chart: allow configuring existingSecrets for nuxeo.conf sensitive data

Currently, the credentials for third-party services can only be configured as plain text in the chart values, enfing up in a ConfigMap mounted as a partial nuxeo.conf file volume.
See for instance the credentials for:

• MongoDB
• Elasticsearch

Let's allow configuring existingSecrets to rely on Kubernetes secrets instead.
We could have one or several secret volumes, such as in the MongoDB Bitnami chart, then mount this/these volume(s) as partial nuxeo.conf files, as we already do with the chart's ConfigMaps.

Or, use environment variables set from Kubernetes secret(s).

We'll have to rework the way we handle nuxeo.conf properties such as "nuxeo.mongodb.server", "elasticsearch.restClient.username", "elasticsearch.restClient.password", etc.

Also, it seems that we are not handling Kafka SSL authentication.