Because of hardcoding name as the 'admin' was impossible to
use administrative panel with a custom administrative role name.
This fix replaces hardcoding the name of the administrative role
with RBAC policy check.
DocImpact
Related commit: https://review.openstack.org/#/c/123745/
Change-Id: I05c8fc750c56f6f6bb49a435662e821eb0d6ba30
Closes-Bug: #1161144