Ability to set the METADATA_FOLDER in long-lived hop-server

To support the deployment of webservices in containerized long-lived hop-server, it would be nice to be able to set the option metadata_folder that will be used in hop-server.xml through an environment variable

<hop-server-config>
  <hop-server>
    ...
  </hop-server>
  <metadata_folder>/home/hop/project/services/metadata</metadata_folder>

</hop-server-config>

For now, containerized long-lived hop-server only support few options, and there is no alternative to force the usage of a given hop-server.xml by the container as the starting code generate its own config file in /tmp/hop-server.xml