Add access to /v3/auth/system

[Radomir Dopieralski]

In order to support the system scope tokens in Horizon, we need access to the auth/system endpoint, so that we can check whether a given user has access to the system scope or not.

[Radomir Dopieralski (deshipu)]

Here are more details.

I order to be able to switch scopes in Horizon, we need to be able to created a Token scoped to System Scope from an unscoped token. Here is an example of an API call with scope information included:

https://docs.openstack.org/api-ref/identity/v3/index.html?expanded=token-authentication-with-scoped-authorization-detail#token-authentication-with-scoped-authorization

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/python-keystoneclient 4.4.0 release.