cherry-pick: https://review.openstack.org/#/c/563679/
jira: OS-6474

Add an admin service account and give it the
cluster role. It can be used for access apps
with token authentication like the
kubernetes-dashboard.

Remove the cluster role from the dashboard service account.

Change-Id: I7980c0e72b0d71921e42af7338d02b8a1e563c34
Closes-Bug: #1766284
(cherry picked from commit 91d5229b)