Horizon should use the authorization API in keystone to build authorization targets for users

During the Xena PTG we discussed how to continue integrating the secure RBAC effort into Horizon [0].

One improvement we agreed upon was for Horizon to use the user's unscoped token to fetch authorization scopes (GET /v3/auth/projects, GET /v3/auth/domains, GET /v3/auth/system) [1].

Then horizon can present a list of targets and rescope tokens similar to what it does today. Additionally, this is a good way to start integrating support for system-scoped tokens into Horizon, which horizon will need in the future when it's required by policy.

[0] https://etherpad.opendev.org/p/policy-popup-xena-ptg
[1] https://docs.openstack.org/api-ref/identity/v3/?expanded=get-available-project-scopes-detail#authentication-and-token-management