This PR adds an option to disable deployment of the demo course.
*Sandbox URL*: TBD - sandbox is being provisioned.
*Merge deadline*: "None" if there's no rush, "ASAP" if it's critical, or provide a specific date if there is one.
1. set `DEMO_ROLE_ENABLED: false`
2. run the deployment
3. verify that the demo course is not present
*Author notes and concerns*:
[ ] @pomegranited
[ ] edX reviewer[s] TBD