Harmonize paths declaration in Arnold

[sampaccoud]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
In openedx-docker, some directories are set by default and must be present in the corresponding volume before deploying edxapp.

For example, ORA2_FILEUPLOAD_ROOT is set to /edx/app/edxapp/data/openassessment_submissions, so an openassessment_submissions directory must pre-exist in Arnold's edxapp-v-data volume that is mounted on /edx/app/edxapp/data.

In Arnold, things are not fully coherent:

• The /edx/app/edxapp/data path is hardcoded whereas the /edx/var/edxapp/export path is set via the edxapp_export_volume_path variable.

• The grades download root path is forced, whereas the ORA2_FILEUPLOAD_ROOT mentionned above is not...

In PR #432, we have introduced a mechanism to let Arnold take care of creating the directories required by edxapp. We now have a new variable required_directories that defines which directories must be created. They are harcoded and must be kept in sync with the corresponding settings in openedx-docker...

Describe the solution you'd like

• Explicitly force in Arnold all paths that are configurable in openedx-docker (like it is done for the grades download root path),
• Explicitly force in Arnold all volume mounts (like it is done for edxapp_export_volume_path),