Fix legacy admin in RBAC tests

The legacy admin continues to work as it is with the sRBAC [0]
if os_system_admin is allowed, os_admin should be allowed.

[0] https://governance.openstack.org/tc/goals/selected/\
    consistent-and-secure-rbac.html\#legacy-admin-continues-to-work-as-it-is

Depends-On: https://review.opendev.org/c/openstack/octavia/+/875620

Change-Id: I10d497a4b4e3a3b21cb24dba73d5074a71a3d381
1 file changed