Switch to use primary credential in volume backup tests

Not all the apis in tempest/api/volume/admin/test_volumes_backup.py
should use admin credential, some apis could switch to use primary
credential. See [1].

[1] https://github.com/openstack/cinder/blob/master/cinder/policies/backups.py

Change-Id: Ied77f68c982f3a8445956ad1a4ea47c3795d3294
diff --git a/tempest/api/volume/admin/test_volumes_backup.py b/tempest/api/volume/admin/test_volumes_backup.py
index 375aacb..35eea11 100644
--- a/tempest/api/volume/admin/test_volumes_backup.py
+++ b/tempest/api/volume/admin/test_volumes_backup.py
@@ -59,9 +59,7 @@
         volume = self.create_volume()
         # Create backup
         backup_name = data_utils.rand_name(self.__class__.__name__ + '-Backup')
-        backup = (self.create_backup(backup_client=self.admin_backups_client,
-                                     volume_id=volume['id'],
-                                     name=backup_name))
+        backup = self.create_backup(volume_id=volume['id'], name=backup_name)
         self.assertEqual(backup_name, backup['name'])
 
         # Export Backup
@@ -103,17 +101,16 @@
         self.assertIn(new_id, [b['id'] for b in backups])
 
         # Restore backup
-        restore = self.admin_backups_client.restore_backup(
-            backup['id'])['restore']
-        self.addCleanup(self.admin_volume_client.delete_volume,
+        restore = self.backups_client.restore_backup(backup['id'])['restore']
+        self.addCleanup(self.volumes_client.delete_volume,
                         restore['volume_id'])
         self.assertEqual(backup['id'], restore['backup_id'])
-        waiters.wait_for_volume_resource_status(self.admin_volume_client,
+        waiters.wait_for_volume_resource_status(self.volumes_client,
                                                 restore['volume_id'],
                                                 'available')
 
         # Verify if restored volume is there in volume list
-        volumes = self.admin_volume_client.list_volumes()['volumes']
+        volumes = self.volumes_client.list_volumes()['volumes']
         self.assertIn(restore['volume_id'], [v['id'] for v in volumes])
         waiters.wait_for_volume_resource_status(self.admin_backups_client,
                                                 import_backup['id'],
@@ -126,12 +123,10 @@
         # Create a backup
         backup_name = data_utils.rand_name(
             self.__class__.__name__ + '-Backup')
-        backup = self.create_backup(backup_client=self.admin_backups_client,
-                                    volume_id=volume['id'],
-                                    name=backup_name)
+        backup = self.create_backup(volume_id=volume['id'], name=backup_name)
         self.assertEqual(backup_name, backup['name'])
         # Reset backup status to error
         self.admin_backups_client.reset_backup_status(backup_id=backup['id'],
                                                       status="error")
-        waiters.wait_for_volume_resource_status(self.admin_backups_client,
+        waiters.wait_for_volume_resource_status(self.backups_client,
                                                 backup['id'], 'error')