Adds `HOTG_ADDITIONAL_INSTANCE_TYPES`, which can be used to expand the list of instance types allowed when creating clusters on Asgard.
Also adds the `supervisor` role as a meta requirement, as this is required to run the `hotg` role.
*Merge deadline*: None
This is the deployment process we used for the updated Campus admin server, so this change can be verified by checking the [olive admin instance type list](http://admin.edx-flatu.org/eu-west-1/instanceType/list).
1. Add an ansible variable like:
size: 'Extra Large'
1. Deploy asgard using the [vpc_admin.yml](https://github.com/edx/configuration/blob/master/playbooks/vpc_admin.yml) playbook.
1. Check the Asgard EC2 > Instance Types list to ensure the additional types are added.
*Author notes and concerns*:
1. Simply installing Asgard on a fresh instance added some new instance types (e.g. `c5.xlarge`), but the `t3.xlarge` type was not added. So this change ensures that it is always added, since we rely on it for olive's `edxapp` cluster.
[ ] @swalladge
[ ] edX reviewer[s] TBD
Configuration Pull Request
Make sure that the following steps are done before merging:
[ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
[ ] Are you adding any new default values that need to be overridden when this change goes live? If so:
[ ] Update the appropriate internal repo (be sure to update for all our environments)
[ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.
[ ] Add an entry to the CHANGELOG.
[ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
[ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?