Add support for system scope tokens

[Radomir Dopieralski]

In order to use system-scope tokens in Horizon, we need a way of converting an unscoped token that a user got when they first logged in into a token with system scope. As far as I can tell, so far we can only make project- and domain-scope tokens.

[Radomir Dopieralski (deshipu)]

Here is an example of an API call to get a system scope: https://docs.openstack.org/api-ref/identity/v3/?expanded=password-authentication-with-scoped-authorization-detail#system-scoped-example

[Radomir Dopieralski (deshipu)]

It looks like at least part of the code necessary is already there:

https://github.com/openstack/keystoneauth/blob/master/keystoneauth1/identity/v3/base.py#L166-L174

What is missing is an argument for Token constructor that we could pass to make that code run.