Document how to setup RBAC with Keystone Authentication

[dims]

See note from Saverio on his blog post : https://cloudblog.switch.ch/2018/02/02/openstack-keystone-authentication-for-your-kubernetes-cluster/

I will be correctly authenticated by keystone that will verify my identity, but I will have no authorization to do anything:

[dims]

cc @zioproto

[dims]

cc @Fedosin

[zioproto]

How can I assign this to me ? I will propose a PR for the documentation.