waiters.py: raise BackupException defined in tempest/lib

This patch is to fix the issue which described in the bug #1632410

A timeout is being raised on a volume backup test scenario
which could be that because the volume backup is slow or
we tripped a cinder bug. The real issue is that wait loop
for backups in tempest is referencing a non-existent exception class,
"VolumeBackupException".

Change-Id: I2d74afc29cd52da7f3642bf56ea896c92cfccdca
Closes-bug: #1632410
diff --git a/tempest/common/waiters.py b/tempest/common/waiters.py
index fa951b5..c1942d6 100644
--- a/tempest/common/waiters.py
+++ b/tempest/common/waiters.py
@@ -221,7 +221,7 @@
         body = client.show_backup(backup_id)['backup']
         backup_status = body['status']
         if backup_status == 'error' and backup_status != status:
-            raise exceptions.VolumeBackupException(backup_id=backup_id)
+            raise lib_exc.VolumeBackupException(backup_id=backup_id)
 
         if int(time.time()) - start >= client.build_timeout:
             message = ('Volume backup %s failed to reach %s status '