feat: remove usage of private.yml file

Description

As of Tutor v17.0.0, adding of packages using private.txt is no longer supported

Currently in Grove, users could include private packages using the OPENEDX_PRIVATE_REQUIREMENTS config. Grove would then use private.txt to get these private packages installed in LMS/CMS containers.

With this PR Grove would instead use the mounts add tutor command to get these private packages installed. For any instance currently using OPENEDX_PRIVATE_REQUIREMENTS to install private packages, this MR would not cause any disruptions.

Supporting information

Link to other information about the change, such as GitLab issues, GitHub issues, forum discussions. Be sure to check they are publicly readable, or if not, repeat the information here.

Testing instructions

Steps to test the changes:

1. Check this commit and this corresponding pipeline, where the sysadmin plugin was added as a private requirement.
2. Go to the sysadmin url of the instance.
3. Verify that sysadmin was correctly installed.

Dependencies

List the dependencies required for this change, if any. Do not forget to link grove-template merge request here if that's affected by this change.

Screenshots

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Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

Private Ref : BB-8418