Add waiter to test_volume_backup_delete

Add waiter to test_volume_backup_delete test to prevent badRequest
error caused by invalid backup status.

Change-Id: Id4c1e3d8665458cf1ffa44da21c18d62468472fb
diff --git a/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py b/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
index 25c8504..2861531 100644
--- a/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
+++ b/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
@@ -100,6 +100,8 @@
             volume_id=self.volume['id'])['backup']
         self.addCleanup(test_utils.call_and_ignore_notfound_exc,
                         self.backups_client.delete_backup, backup['id'])
+        waiters.wait_for_volume_resource_status(self.os_admin.backups_client,
+                                                backup['id'], 'available')
 
         self.rbac_utils.switch_role(self, toggle_rbac_role=True)
         self.backups_client.delete_backup(backup['id'])