Support cross-domain authentication in OpenStack provider

[embik]

What would you like to be added? / User Story:

As mentioned in kubermatic/kubermatic#12543, we would like to support cross-domain authentication in OpenStack - this means that a user from one OpenStack domain can manage an OpenStack project in another domain.

This issue is mostly to track and implement whatever solution is implemented in the linked issue, adjusting the api and frontend to what the "backend" implementation of this feature would look like. Since implementation design for this is not fully clear and requires investigation as part of the kubermatic/kubermatic issue, there are no more details at this time.