Fix PTR API tests for scoped tokens

The PTR API tests were not using scoped tokens even though a test
job had scoped tokens enabled. This was not caught because the scoped
token patch for designate has not yet merged.
This patch updates the PTR API tests to use a scoped token for
admin if scoped tokens are enabled in the test run.

Change-Id: I259e7b94aae2e7794dd2514bb50cdc811c518845
diff --git a/designate_tempest_plugin/tests/api/v2/test_ptrs.py b/designate_tempest_plugin/tests/api/v2/test_ptrs.py
index 4303cce..b094a5d 100644
--- a/designate_tempest_plugin/tests/api/v2/test_ptrs.py
+++ b/designate_tempest_plugin/tests/api/v2/test_ptrs.py
@@ -37,7 +37,7 @@
 
 class DesignatePtrRecord(BasePtrTest, tempest.test.BaseTestCase):
 
-    credentials = ['primary', 'admin']
+    credentials = ['primary', 'admin', 'system_admin']
 
     @classmethod
     def setup_credentials(cls):
@@ -48,8 +48,11 @@
     @classmethod
     def setup_clients(cls):
         super(DesignatePtrRecord, cls).setup_clients()
+        if CONF.enforce_scope.designate:
+            cls.admin_ptr_client = cls.os_system_admin.dns_v2.PtrClient()
+        else:
+            cls.admin_ptr_client = cls.os_admin.dns_v2.PtrClient()
         cls.primary_ptr_client = cls.os_primary.dns_v2.PtrClient()
-        cls.admin_ptr_client = cls.os_admin.dns_v2.PtrClient()
         cls.primary_floating_ip_client = cls.os_primary.floating_ips_client
 
     def _set_ptr(self, ptr_name=None, ttl=None, description=None,