Reuse tempest to create admin client manager

The previous code doesn't support PreProvisionedCredentialProvider, it
was getting admin credentials from tempest config file which are not set
when using test_accounts_file.

Change-Id: Ia34d08ad659b095a114c27d6d596507f7922149a
diff --git a/patrole_tempest_plugin/rbac_rule_validation.py b/patrole_tempest_plugin/rbac_rule_validation.py
index 93c9a1d..9334702 100644
--- a/patrole_tempest_plugin/rbac_rule_validation.py
+++ b/patrole_tempest_plugin/rbac_rule_validation.py
@@ -356,9 +356,11 @@
     else:
         formatted_target_data = _format_extra_target_data(
             test_obj, extra_target_data)
+        policy_authority.PolicyAuthority.os_admin = test_obj.os_admin
         authority = policy_authority.PolicyAuthority(
             project_id, user_id, service,
             extra_target_data=formatted_target_data)
+
     is_allowed = authority.allowed(rule, roles)
 
     if is_allowed: