Fixes volumes cleanup in base class

The tearDown method was not cleaning up the environment but failing
silently because of 'pass' statement

Change-Id: I75dde195ff162d0be27d971a551d3618c852e688
Fixes: bug #1172497
diff --git a/tempest/tests/volume/base.py b/tempest/tests/volume/base.py
index 00e8668..978ec53 100644
--- a/tempest/tests/volume/base.py
+++ b/tempest/tests/volume/base.py
@@ -149,13 +149,13 @@
     def clear_volumes(cls):
         for volume in cls.volumes:
             try:
-                cls.volume_client.delete_volume(volume['id'])
+                cls.volumes_client.delete_volume(volume['id'])
             except Exception:
                 pass
 
         for volume in cls.volumes:
             try:
-                cls.servers_client.wait_for_resource_deletion(volume['id'])
+                cls.volumes_client.wait_for_resource_deletion(volume['id'])
             except Exception:
                 pass