Allow to provide additional files to Nuxeo, typically Packages

For now, to install a Nuxeo Package with the Nuxeo Helm chart, it requires to build a custom Docker image embedding the package.

When this image is not to be distributed, this is overkill and creates some issues.

It would be useful that the deployment.yaml offers a solution to provide local Nuxeo Packages (ie: not to download from the Marketplace).

A combination of configmap and volume mounts could be fine.

Like https://github.com/helm/charts/blob/master/stable/mongodb/templates/deployment-standalone.yaml#L196 for instance.