javelin: Detach the volume before destruction

In Icehouse, a volume needs to be detached before it can be deleted.

Closes-Bug:  #1375476

Change-Id: Ic1c8e79fc1efa46ac9a70e288166cd5e0d823cfa
diff --git a/tempest/cmd/javelin.py b/tempest/cmd/javelin.py
index bf0cc70..27e388d 100755
--- a/tempest/cmd/javelin.py
+++ b/tempest/cmd/javelin.py
@@ -525,7 +525,8 @@
     for volume in volumes:
         client = client_for_user(volume['owner'])
         volume_id = _get_volume_by_name(client, volume['name'])['id']
-        r, body = client.volumes.delete_volume(volume_id)
+        client.volumes.detach_volume(volume_id)
+        client.volumes.delete_volume(volume_id)
 
 
 def attach_volumes(volumes):