BB-1551: Set sane defaults for Mongo Auth URL

[giovannicimolin]

Configuration Pull Request

Make sure that the following steps are done before merging

• A devops team member has commented with 👍
• are you adding any new default values that need to be overridden when this goes live?
  • Open a ticket (DEVOPS) to make sure that they have been added to secure vars.
  • Add an entry to the CHANGELOG.
• Are you adding/updating any variables that need to be added/updated to a specific configuration-secure repo?

[xitij2000]

Looks good to go. I don't think the change to the edxapp role is necessary, but shouldn't hurt either, it's just reinforcing a default.