Fix display_name of volume for test_volumes_list

In test_volumes_list, three volumes are created for some tests.
These display_name seems wrong like "volume-%s1393726625json".
The interface name should be inserted after "volume-", but it is
appended to the tail.
This patch fixes this volume name.

Change-Id: Iee55d2405e888569455a2325fe6b2843ddf9de9d
diff --git a/tempest/api/compute/volumes/test_volumes_list.py b/tempest/api/compute/volumes/test_volumes_list.py
index f214641..f54e9b3 100644
--- a/tempest/api/compute/volumes/test_volumes_list.py
+++ b/tempest/api/compute/volumes/test_volumes_list.py
@@ -43,9 +43,8 @@
         cls.volume_list = []
         cls.volume_id_list = []
         for i in range(3):
-            v_name = data_utils.rand_name('volume-%s')
+            v_name = data_utils.rand_name('volume-%s' % cls._interface)
             metadata = {'Type': 'work'}
-            v_name += cls._interface
             try:
                 resp, volume = cls.client.create_volume(size=1,
                                                         display_name=v_name,