feat: add multiple SSO tenants during devstack provisioning

The goal is ultimately to support integration testing logistration
flows which depend on many different enterprise SSO configurations,
including multi-tenant configurations.

Changes directly in service of that goal:

- Update provision-tpa.py to provision multiple SSO tenants (gryffindor, slytherin).
- Add devstack_api.py helpers for SAML providers, enterprise branding, etc.
- Add ENT-11568 integration-test script.

Cleanup work along the way:

- Eliminated legacy code that provisioned several redundant worker and
  admin users (devstack already provisions them).
- Add `assign_system_wide_enterprise_role` management command to finally fix this:
  https://github.com/edx/devstack/blob/4e42eeda/provision-enterprise-access.sh#L25
- [Performance] Add --no-create-users flag to seed_enterprise_devstack_data command

ENT-11568